Shishi

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
copy of the license is included in the section entitled “GNU Free
Documentation License”.

1 Introduction

Shishi is an implementation of the Kerberos 5 network authentication
system, as specified in RFC 4120. Shishi can be used to authenticate
users in distributed systems.

Shishi contains a library ('libshishi') that can be used by
application developers to add support for Kerberos 5. Shishi contains
a command line utility ('shishi') that is used by users to acquire and
manage tickets (and more). The server side, a Key Distribution
Center, is implemented by 'shishid'. Of course, a manual documenting
usage aspects as well as the programming API is included.

Shishi is internationalized; error and status messages can be
translated into the users' language; user name and passwords can be
converted into any available character set (normally including
ISO-8859-1 and UTF-8) and also be processed using an experimental
Stringprep profile.

Most, if not all, of the widely used encryption and checksum types are
supported, such as 3DES, AES, ARCFOUR and HMAC-SHA1.

Shishi is developed for the GNU/Linux system, but runs on over 20
platforms including most major Unix platforms and Windows, and many
kind of devices including iPAQ handhelds and S/390 mainframes.

Shishi is free software licensed under the GNU General Public License
version 3.0 or later.

1.1 Getting Started

This manual documents the Shishi application and library programming
interface. All commands, functions and data types provided by Shishi
are explained.

The reader is assumed to possess basic familiarity with network
security and the Kerberos 5 security system.

This manual can be used in several ways. If read from the beginning
to the end, it gives a good introduction into the library and how it
can be used in an application. Forward references are included where
necessary. Later on, the manual can be used as a reference manual to
get just the information needed about any particular interface of the
library. Experienced programmers might want to start looking at the
examples at the end of the manual, and then only read up on those parts
of the interface which are unclear.

1.2 Features and Status

Shishi might have a couple of advantages over other packages doing a
similar job.

It's Free Software

Anybody can use, modify, and redistribute it under the terms of the
GNU General Public License version 3.0 or later.

It's thread-safe

The library uses no global variables.

It's internationalized

It handles non-ASCII username and passwords, and user visible strings
used in the library (error messages) can be translated into the users'
language.

It's portable

It should work on all Unix like operating systems, including Windows.

Shishi is far from feature complete, it is not even a full RFC 1510
implementation yet. However, some basic functionality is implemented.
A few implemented feature are mentioned below.

Initial authentication (AS) from raw key or password.
This step is typically used to acquire a ticket granting ticket and,
less commonly, a server ticket.

Subsequent authentication (TGS).
This step is typically used to acquire a server ticket, by
authenticating yourself using the ticket granting ticket.

Client-Server authentication (AP).
This step is used by clients and servers to prove to each other who
they are, using negotiated tickets.

Integrity protected communication (SAFE).
This step is used by clients and servers to exchange integrity
protected data with each other. The key is typically agreed on using
the Client-Server authentication step.

Ticket cache, supporting multiple principals and realms.
As tickets have a life time of typically several hours, they are
managed in disk files. There can be multiple ticket caches, and each
ticket cache can store tickets for multiple clients (users), servers,
encryption types, etc. Functionality is provided for locating the
proper ticket for every use.

Telnet client and server.
This is used to remotely login to other machines, after authenticating
yourself with a ticket.

PAM module.
This is used to login locally on a machine.

KDC addresses located using DNS SRV RRs.

Modularized low-level crypto interface.
Currently Gnulib and Libgcrypt are supported. If you wish to add
support for another low-level cryptographic library, you only have to
implement a few APIs for DES, AES, MD5, SHA1, HMAC, etc. Look at
gl/gc-gnulib.c or gl/gc-libgcrypt.c as a starting
pointer.

The following table summarize what the current objectives are (i.e.,
the todo list) and an estimate on how long it will take to implement
the feature, including some reasonable startup-time to get familiar
with Shishi in general. If you like to start working on anything,
please let me know so work duplication can be avoided.

Parse /etc/krb5.keytab to extract keys to use for telnetd etc (week)

Cross-realm support (week).

PKINIT (use libksba, weeks)

Finish GSSAPI support via GSSLib (weeks)
Shishi will not support GSSLib natively, but a separate project
“GSSLib” is under way to produce a generic GSS implementation, and
it will use Shishi to implement the Kerberos 5 mechanism.

Port to cyclone (cyclone need to mature first)

Modularize ASN.1 library so it can be replaced (days).
Almost done, all ASN.1 functionality is found in lib/asn1.c, although
the interface is rather libtasn1 centric.

KDC (initiated, weeks)

LDAP backend for Shisa.

Set/Change password protocol (weeks?)

Port applications to use Shishi (indefinite)

Finish server-realm stuff

Improve documentation

Improve internationalization

Add AP-REQ replay cache (week).

Study benefits by introducing a PA-TGS-REP.
This would provide mutual authentication of the KDC in a way that is
easier to analyze. Currently the mutual authentication property is
only implicit from successful decryption of the KDC-REP and the 4 byte
nonce.

GUI applet for managing tickets.
This is supported via the ticket-applet, of which a Shishi port is
published on the Shishi home page.

Authorization library (months?)
The shishi_authorized_p() is not a good solution, better would be to
have a generic and flexible authorization library. Possibly based on
S-EXP's in tickets? Should support non-Kerberos uses as well, of
course.

Proof read manual.

X.500 support, including DOMAIN-X500-COMPRESS.
I will accept patches that implement this, if it causes minimal
changes to the current code.

1.3 Overview

Kerberos provides a means of verifying the identities of principals,
(e.g., a workstation user or a network server) on an open
(unprotected) network. This is accomplished without relying on
authentication by the host operating system, without basing trust on
host addresses, without requiring physical security of all the hosts
on the network, and under the assumption that packets traveling along
the network can be read, modified, and inserted at will. (Note,
however, that many applications use Kerberos' functions only upon the
initiation of a stream-based network connection, and assume the
absence of any "hijackers" who might subvert such a connection. Such
use implicitly trusts the host addresses involved.) Kerberos performs
authentication under these conditions as a trusted third- party
authentication service by using conventional cryptography, i.e.,
shared secret key. (shared secret key - Secret and private are often
used interchangeably in the literature. In our usage, it takes two
(or more) to share a secret, thus a shared DES key is a secret key.
Something is only private when no one but its owner knows it. Thus,
in public key cryptosystems, one has a public and a private key.)

The authentication process proceeds as follows: A client sends a
request to the authentication server (AS) requesting "credentials" for
a given server. The AS responds with these credentials, encrypted in
the client's key. The credentials consist of 1) a "ticket" for the
server and 2) a temporary encryption key (often called a "session
key"). The client transmits the ticket (which contains the client's
identity and a copy of the session key, all encrypted in the server's
key) to the server. The session key (now shared by the client and
server) is used to authenticate the client, and may optionally be used
to authenticate the server. It may also be used to encrypt further
communication between the two parties or to exchange a separate
sub-session key to be used to encrypt further communication.

The implementation consists of one or more authentication servers
running on physically secure hosts. The authentication servers
maintain a database of principals (i.e., users and servers) and their
secret keys. Code libraries provide encryption and implement the
Kerberos protocol. In order to add authentication to its
transactions, a typical network application adds one or two calls to
the Kerberos library, which results in the transmission of the
necessary messages to achieve authentication.

The Kerberos protocol consists of several sub-protocols (or
exchanges). There are two methods by which a client can ask a
Kerberos server for credentials. In the first approach, the client
sends a cleartext request for a ticket for the desired server to the
AS. The reply is sent encrypted in the client's secret key. Usually
this request is for a ticket-granting ticket (TGT) which can later be
used with the ticket-granting server (TGS). In the second method, the
client sends a request to the TGS. The client sends the TGT to the
TGS in the same manner as if it were contacting any other application
server which requires Kerberos credentials. The reply is encrypted in
the session key from the TGT.

Once obtained, credentials may be used to verify the identity of the
principals in a transaction, to ensure the integrity of messages
exchanged between them, or to preserve privacy of the messages. The
application is free to choose whatever protection may be necessary.

To verify the identities of the principals in a transaction, the
client transmits the ticket to the server. Since the ticket is sent
"in the clear" (parts of it are encrypted, but this encryption doesn't
thwart replay) and might be intercepted and reused by an attacker,
additional information is sent to prove that the message was
originated by the principal to whom the ticket was issued. This
information (called the authenticator) is encrypted in the session
key, and includes a timestamp. The timestamp proves that the message
was recently generated and is not a replay. Encrypting the
authenticator in the session key proves that it was generated by a
party possessing the session key. Since no one except the requesting
principal and the server know the session key (it is never sent over
the network in the clear) this guarantees the identity of the client.

The integrity of the messages exchanged between principals can also be
guaranteed using the session key (passed in the ticket and contained
in the credentials). This approach provides detection of both replay
attacks and message stream modification attacks. It is accomplished
by generating and transmitting a collision-proof checksum (elsewhere
called a hash or digest function) of the client's message, keyed with
the session key. Privacy and integrity of the messages exchanged
between principals can be secured by encrypting the data to be passed
using the session key passed in the ticket, and contained in the
credentials.

1.4 Cryptographic Overview

Shishi implements several of the standard cryptographic primitives.
In this section we give the names of the supported encryption suites,
and some notes about them, and their associated checksum suite.

Statements such as “it is weak” should be read as meaning that there
is no credible security analysis of the mechanism available, and/or
that should an attack be published publicly, few people would likely
be surprised. Also keep in mind that the key size mentioned is the
actual key size, not the effective key space as far as a brute force
attack is concerned.

As you may infer from the descriptions, there is currently no
encryption algorithm and only one checksum algorithm that inspire
great confidence in its design. Hopefully this will change over time.

NULL

NULL is a dummy encryption suite for debugging. Encryption and
decryption are identity functions. No integrity protection. It is
weak. It is associated with the NULL checksum.

arcfour-hmac

arcfour-hmac-exp

arcfour-hmac-* are a proprietary stream cipher with 56 bit
(arcfour-hmac-exp) or 128 bit (arcfour-hmac) keys, used
in a proprietary way described in an expired IETF draft
draft-brezak-win2k-krb-rc4-hmac-04.txt. Deriving keys from
passwords is supported, and is done by computing a message digest
(MD4) of a 16-bit Unicode representation of the ASCII password, with
no salt. Data is integrity protected with a keyed hash (HMAC-MD5),
where the key is derived from the base key in a creative way. It is
weak. It is associated with the arcfour-hmac-md5 checksum.

des-cbc-none

des-cbc-none is DES encryption and decryption with 56 bit keys
and 8 byte blocks in CBC mode, using a zero IV. The keys can be
derived from passwords by an obscure application specific algorithm.
It is weak, because it offers no integrity protection. This is
typically only used by RFC 1964 GSS-API implementations (which try to
protect integrity using an ad-hoc solution). It is associated with
the NULL checksum.

des-cbc-crc

des-cbc-crc is DES encryption and decryption with 56 bit keys
and 8 byte blocks in CBC mode, using the key as IV (see Key as initialization vector). The keys can be derived from passwords by an
obscure application specific algorithm. Data is integrity protected
with an unkeyed but encrypted CRC32-like checksum. It is
weak. It is associated with the rsa-md5-des checksum.

des-cbc-md4

des-cbc-md4 is DES encryption and decryption with 56 bit keys
and 8 byte blocks in CBC mode, using a zero IV. The keys can be
derived from passwords by an obscure application specific algorithm.
Data is integrity protected with an unkeyed but encrypted MD4 hash.
It is weak. It is associated with the rsa-md4-des checksum.

des-cbc-md5

des-cbc-md5 is DES encryption and decryption with 56 bit keys
and 8 byte blocks in CBC mode, using a zero IV. The keys can be
derived from passwords by an obscure application specific algorithm.
Data is integrity protected with an unkeyed but encrypted MD5 hash.
It is weak. It is associated with the rsa-md5-des checksum.
This is the strongest RFC 1510 interoperable encryption mechanism.

des3-cbc-none

des3-cbc-none is DES encryption and decryption with three 56
bit keys (effective key size 112 bits) and 8 byte blocks in CBC mode.
The keys can be derived from passwords by the same algorithm as
des3-cbc-sha1-kd. It is weak, because it offers no integrity
protection. This is typically only used by GSS-API implementations
(which try to protect integrity using an ad-hoc solution) for
interoperability with some existing Kerberos GSS implementations. It
is associated with the NULL checksum.

des3-cbc-sha1-kd

des3-cbc-sha1-kd is DES encryption and decryption with three 56
bit keys (effective key size 112 bits) and 8 byte blocks in CBC mode.
The keys can be derived from passwords by a algorithm based on the
paper "A Better Key Schedule For DES-like Ciphers"
2 by
Uri Blumenthal and Steven M. Bellovin (it is not clear if the
algorithm, and the way it is used, is used by any other protocols,
although it seems unlikely). Data is integrity protected with a keyed
SHA1 hash in HMAC mode. It has no security proof, but is assumed to
provide adequate security in the sense that knowledge on how to crack
it is not known to the public. Note that the key derivation function
is not widely used outside of Kerberos, hence not widely studied. It
is associated with the hmac-sha1-des3-kd checksum.

aes128-cts-hmac-sha1-96

aes256-cts-hmac-sha1-96

aes128-cts-hmac-sha1-96 and aes256-cts-hmac-sha1-96 is
AES encryption and decryption with 128 bit and 256 bit key,
respectively, and 16 byte blocks in CBC mode with Cipher Text
Stealing. Cipher Text Stealing means data length of encrypted data is
preserved (pure CBC add up to 7 pad characters). The keys can be
derived from passwords with RSA Laboratories PKCS#5 Password Based Key
Derivation Function
23,
which is allegedly provably secure in a random oracle model. Data is
integrity protected with a keyed SHA1 hash, in HMAC mode, truncated to
96 bits. There is no security proof, but the schemes are assumed to
provide adequate security in the sense that knowledge on how to crack
them is not known to the public. Note that AES has yet to receive the
test of time, and the AES cipher encryption mode (CBC with Ciphertext
Stealing, and a non-standard IV output) is not widely standardized
(hence not widely studied). It is associated with the
hmac-sha1-96-aes128 and hmac-sha1-96-aes256 checksums,
respectively.

The protocol do not include any way to negotiate which checksum
mechanisms to use, so in most cases the associated checksum will be
used. However, checksum mechanisms can be used with other encryption
mechanisms, as long as they are compatible in terms of key format etc.
Here are the names of the supported checksum mechanisms, with some
notes on their status and the compatible encryption mechanisms. They
are ordered by increased security as perceived by the author.

NULL

NULL is a dummy checksum suite for debugging. It provides no
integrity. It is weak. It is compatible with the NULL
encryption mechanism.

arcfour-hmac-md5

arcfour-hmac-md5 is a keyed HMAC-MD5 checksum computed on a MD5
message digest, in turn computed on a four byte message type indicator
concatenated with the application data. (The arcfour
designation is thus somewhat misleading, but since this checksum
mechanism is described in the same document as the arcfour
encryption mechanisms, it is not a completely unnatural designation.)
It is weak. It is compatible with all encryption mechanisms.

rsa-md4

rsa-md4 is a unkeyed MD4 hash computed over the message. It is
weak, because it is unkeyed. However applications can, with care, use
it non-weak ways (e.g., by including the hash in other messages that
are protected by other means). It is compatible with all encryption
mechanisms.

rsa-md4-des

rsa-md4-des is a DES CBC encryption of one block of random data
and a unkeyed MD4 hash computed over the random data and the message
to integrity protect. The key used is derived from the base protocol
key by XOR with a constant. It is weak. It is compatible with the
des-cbc-crc, des-cbc-md4, des-cbc-md5 encryption
mechanisms.

rsa-md5

rsa-md5 is a unkeyed MD5 hash computed over the message. It is
weak, because it is unkeyed. However applications can, with care, use
it non-weak ways (e.g., by including the hash in other messages that
are protected by other means). It is compatible with all encryption
mechanisms.

rsa-md5-des

rsa-md5-des is a DES CBC encryption of one block of random data
and a unkeyed MD5 hash computed over the random data and the message
to integrity protect. The key used is derived from the base protocol
key by XOR with a constant. It is weak. It is compatible with the
des-cbc-crc, des-cbc-md4, des-cbc-md5 encryption
mechanisms.

hmac-sha1-des3-kd

hmac-sha1-des3-kd is a keyed SHA1 hash in HMAC mode computed
over the message. The key is derived from the base protocol by the
simplified key derivation function (similar to the password key
derivation functions of des3-cbc-sha1-kd, which does not appear
to be widely used outside Kerberos and hence not widely studied). It
has no security proof, but is assumed to provide good security. The
weakest part is likely the proprietary key derivation function. It is
compatible with the des3-cbc-sha1-kd encryption mechanism.

hmac-sha1-96-aes128

hmac-sha1-96-aes256

hmac-sha1-96-aes* are keyed SHA1 hashes in HMAC mode computed
over the message and then truncated to 96 bits. The key is derived
from the base protocol by the simplified key derivation function
(similar to the password key derivation functions of
aes*-cts-hmac-sha1-96, i.e., PKCS#5). It has no security
proof, but is assumed to provide good security. It is compatible with
the aes*-cts-hmac-sha1-96 encryption mechanisms.

Several of the cipher suites have long names that can be hard to
memorize. For your convenience, the following short-hand aliases
exists. They can be used wherever the full encryption names are used.

1.8 Downloading and Installing

The latest version is stored in a file, e.g.,
‘shishi-1.0.2.tar.gz’ where the ‘1.0.2’
indicate the highest version number.

The package is then extracted, configured and built like many other
packages that use Autoconf. For detailed information on configuring
and building it, refer to the INSTALL file that is part of the
distribution archive.

Here is an example terminal session that download, configure, build
and install the package. You will need a few basic tools, such as
‘sh’, ‘make’ and ‘cc’.

1.9 Bug Reports

If you think you have found a bug in Shishi, please investigate it and
report it.

Please make sure that the bug is really in Shishi, and
preferably also check that it hasn't already been fixed in the latest
version.

You have to send us a test case that makes it possible for us to
reproduce the bug.

You also have to explain what is wrong; if you get a crash, or
if the results printed are not good and in that case, in what way.
Make sure that the bug report includes all information you would need
to fix this kind of bug for someone else.

Please make an effort to produce a self-contained report, with
something definite that can be tested or debugged. Vague queries or
piecemeal messages are difficult to act on and don't help the
development effort.

If your bug report is good, we will do our best to help you to get a
corrected version of the software; if the bug report is poor, we won't
do anything about it (apart from asking you to send better bug
reports).

If you think something in this manual is unclear, or downright
incorrect, or if the language needs to be improved, please also send a
note.

1.10 Contributing

If you want to submit a patch for inclusion – from solve a typo you
discovered, up to adding support for a new feature – you should
submit it as a bug report (see Bug Reports). There are some
things that you can do to increase the chances for it to be included
in the official package.

Unless your patch is very small (say, under 10 lines) we require that
you assign the copyright of your work to the Free Software Foundation.
This is to protect the freedom of the project. If you have not
already signed papers, we will send you the necessary information when
you submit your contribution.

For contributions that doesn't consist of actual programming code, the
only guidelines are common sense. Use it.

If you normally code using another coding standard, there is no
problem, but you should use ‘indent’ to reformat the code
(see GNU Indent) before submitting your work.

Use the unified diff format ‘diff -u’.

Return errors.
The only valid reason for ever aborting the execution of the program
is due to memory allocation errors, but for that you should call
‘shishi_xalloc_die’ to allow the application to recover if it
wants to.

Design with thread safety in mind.
Don't use global variables. Don't even write to per-handle global
variables unless the documented behaviour of the function you write is
to write to the per-handle global variable.

Avoid using the C math library.
It causes problems for embedded implementations, and in most
situations it is very easy to avoid using it.

Document your functions.
Use comments before each function headers, that, if properly
formatted, are extracted into Texinfo manuals and GTK-DOC web pages.

2 User Manual

Usually Shishi interacts with you to get some initial authentication
information like a password, and then contacts a server to receive a
so called ticket granting ticket. From now on, you rarely interact
with Shishi directly. Applications that need security services
instruct the Shishi library to use the ticket granting ticket to get
new tickets for various servers. An example could be if you log on to
a host remotely via ‘telnet’. The host usually requires
authentication before permitting you in. The ‘telnet’ client
uses the ticket granting ticket to get a ticket for the server, and
then uses this ticket to authenticate you against the server (typically
the server is also authenticated to you). You perform the initial
authentication by typing shishi at the prompt. Sometimes it
is necessary to supply options telling Shishi what your principal name
(user name in the Kerberos realm) or your realm is. In the example, I
specify the client name simon@JOSEFSSON.ORG.

As you can see, I had a ticket for the server
‘host/latte.josefsson.org’ which was generated by
‘telnet’:ing to that host.

If, for some reason, you want to manually get a ticket for a specific
server, you can use the shishi --server-name command.
Normally, however, the application that uses Shishi will take care of
getting a ticket for the appropriate server, so you normally wouldn't
need to issue this command.

As you can see, I acquired a ticket for ‘user/billg’ with a
‘des-cbc-md4’ (see Cryptographic Overview) encryption key
specified with the ‘--encryption-type’ parameter.

To wrap up this introduction, let us see how you can remove tickets.
You may want to do this if you leave your terminal for lunch or
similar, and don't want someone to be able to copy the file and then
use your credentials. Note that this only destroys the tickets
locally, it does not contact any server telling that these
credentials are no longer valid. So, if someone stole your ticket
file, you must still contact your administrator and have them reset your
account. Simply using this switch is not sufficient.

Since the ‘--server-name’ parameter takes a long string to type,
it is possible to type the server name directly, after the client name.
The following example demonstrates an AS-REQ followed by a TGS-REQ for a
specific server (assuming you did not have any tickets to begin with).

Refer to the reference manual for all available parameters
(see Parameters for shishi). The rest of this section contains
descriptions of more specialized usage modes that can be ignored by
most users.

2.1 Proxiable and Proxy Tickets

At times it may be necessary for a principal to allow a service to
perform an operation on its behalf. The service must be able to take
on the identity of the client, but only for a particular purpose. A
principal can allow a service to take on the principal's identity for
a particular purpose by granting it a proxy.

The process of granting a proxy using the proxy and proxiable flags is
used to provide credentials for use with specific services. Though
conceptually also a proxy, users wishing to delegate their identity in
a form usable for all purpose MUST use the ticket forwarding mechanism
described in the next section to forward a ticket-granting ticket.

The PROXIABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
When set, this flag tells the ticket-granting server that it is OK to
issue a new ticket (but not a ticket-granting ticket) with a different
network address based on this ticket. This flag is set if requested by
the client on initial authentication. By default, the client will
request that it be set when requesting a ticket-granting ticket, and
reset when requesting any other ticket.

This flag allows a client to pass a proxy to a server to perform a
remote request on its behalf (e.g. a print service client can give the
print server a proxy to access the client's files on a particular file
server in order to satisfy a print request).

In order to complicate the use of stolen credentials, Kerberos tickets
are usually valid from only those network addresses specifically
included in the ticket[4]. When granting a proxy, the client MUST
specify the new network address from which the proxy is to be used, or
indicate that the proxy is to be issued for use from any address.

The PROXY flag is set in a ticket by the TGS when it issues a proxy
ticket. Application servers MAY check this flag and at their option
they MAY require additional authentication from the agent presenting
the proxy in order to provide an audit trail.

Here is how you would acquire a PROXY ticket for the service
‘imap/latte.josefsson.org’:

As you noticed, this asked for your password. The reason is that
proxy tickets must be acquired using a proxiable ticket granting
ticket, which was not present. If you often need to get proxy
tickets, you may acquire a proxiable ticket granting ticket from the
start:

2.2 Forwardable and Forwarded Tickets

Authentication forwarding is an instance of a proxy where the service
that is granted is complete use of the client's identity. An example
where it might be used is when a user logs in to a remote system and
wants authentication to work from that system as if the login were
local.

The FORWARDABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
The FORWARDABLE flag has an interpretation similar to that of the
PROXIABLE flag, except ticket-granting tickets may also be issued with
different network addresses. This flag is reset by default, but users
MAY request that it be set by setting the FORWARDABLE option in the AS
request when they request their initial ticket-granting ticket.

This flag allows for authentication forwarding without requiring the
user to enter a password again. If the flag is not set, then
authentication forwarding is not permitted, but the same result can
still be achieved if the user engages in the AS exchange specifying
the requested network addresses and supplies a password.

The FORWARDED flag is set by the TGS when a client presents a ticket
with the FORWARDABLE flag set and requests a forwarded ticket by
specifying the FORWARDED KDC option and supplying a set of addresses
for the new ticket. It is also set in all tickets issued based on
tickets with the FORWARDED flag set. Application servers may choose to
process FORWARDED tickets differently than non-FORWARDED tickets.

If addressless tickets are forwarded from one system to another,
clients SHOULD still use this option to obtain a new TGT in order to
have different session keys on the different systems.

Here is how you would acquire a FORWARDED ticket for the service
‘host/latte.josefsson.org’:

As you noticed, this asked for your password. The reason is that
forwarded tickets must be acquired using a forwardable ticket granting
ticket, which was not present. If you often need to get forwarded
tickets, you may acquire a forwardable ticket granting ticket from the
start:

3 Administration Manual

Here you will learn how to set up, run and maintain the Shishi
Kerberos server. Kerberos is incompatible with the standard Unix
/etc/passwd password database4, therefore the first
step will be to create a Kerberos user database. Shishi's user
database system is called Shisa. Once Shisa has been configured, you can
then start the server and begin issuing Kerberos tickets to your
users. The Shishi server is called shishid. After getting the
server up and running, we discuss how you can set up multiple Kerberos
servers, to increase availability or offer load-balancing. Finally,
we include some information intended for developers, that will enable
you to customize Shisa to use an external user database, such as a
LDAP server or SQL database.

3.1 Introduction to Shisa

The user database part of Shishi is called Shisa. The Shisa library
is independent of the core Shishi library. Shisa is responsible for
storing the name of your realms, the name of your principals (users),
accounting information for the users (i.e., when each account starts to
be valid and when it expires), and the cryptographic keys each user
has. Some Kerberos internal data can also be stored, such as the key
version number, the last dates for when various ticket requests were
made, the cryptographic salt, string-to-key parameters and password
for each user. Not all information need to be stored. For example,
in some situations it is prudent to leave the password field empty, so
that somebody who manages to steal the user database will only be able
to compromise your system, and not any other systems were your user may
have re-used the same password. On the other hand, you may already
be storing the password in your customized database, in which case being
able to change it via the Shisa interface can be useful.

Shisa is a small (a few thousand lines of C code) standalone
library. Shisa does not depend on the Shishi library. Because a user
database with passwords may be useful for other applications as well
(e.g., GNU SASL), it might be separated into its own
project later on. You should keep this in mind, so that you don't
consider writing a Shisa backend for your own database as a purely Shishi
specific project. You can, for example, choose to use the Shisa
interface in your own applications to have a simple interface to your
user database. Your experience and feedback is appreciated if you
have chosen to explore this.

Note that the Shisa database does not expose everything you may want
to know about a user, such as its full human name, telephone number or
even the user's login account name or home directory. It only stores
what is needed to authenticate a peer claiming to be an entity. Thus
it does not make sense to replace your current user database or
/etc/passwd with data derived from the Shisa database.
Instead, it is intended that you write a Shisa backend that exports
some of the information stored in your user database. You may be
able to replace some existing functionality, such as the password
field in /etc/passwd with a Kerberos PAM module, but there is
no requirement for doing so.

3.2 Configuring Shisa

The configuration file for Shisa is typically stored in
/usr/local/etc/shishi/shisa.conf. You do not have to modify
this file, the defaults should be acceptable to first-time users. The
file is used to define where your user database resides, and some
options such as making the database read-only, or whether errors
detected when accessing the database should be ignored. (The latter
could be useful if the server is a remote LDAP server that might
be unavailable, and then you would want to fall back to a local copy
of the database.)

The default will store the user database using directories and files,
rooted by default in /usr/local/var/shishi. You can use
standard file permission settings to control access to the directory
hierarchy. It is strongly recommended to restrict access to the
directory. Storing the directory on local storage, i.e., hard disk or
removable media, is recommended. We discourage placing the database on
a network file system, but realize this can be useful in some situations
(see Multiple servers).

See the reference manual (see Shisa Configuration) for the details
of the configuration file. Again, you are not expected to need to
modify anything unless you are an experienced Shishi administrator.

3.3 Using Shisa

There is a command line interface to the Shisa library, aptly named
shisa. You will use this tool to add, remove, and change
information stored in the database about realms, principals, and keys.
The tool can also be used to “dump” all information in the database,
for backup or debugging purposes. (Currently the output format cannot
be read by any tool, but functionality to do this will be added in the
future, possibly as a read-only file-based Shisa database backend.)

The reference manual (see Parameters for shisa) explains all
parameters, but here we will give you a walk-through of the typical
uses of the tool.

Installing Shishi usually creates a realm with two principals: one
ticket granting ticket for the realm, and one host key for the server.
This is what you typically need to get started, but it doesn't serve
our purposes, so we start by removing the principals and the realm.
To do that, we need to figure out the name of the realm. The
‘--list’ or ‘--dump’ parameters can be used for this. (Most
“long” parameters, like ‘--dump’, have shorter names as well,
in this case ‘-d’, Parameters for shisa).

The realm names are printed at column 0, the principal names are
indented with one ‘TAB’ character (aka ‘\t’ or ASCII 0x09
Horizontal Tabulation), and the information about each principal is
indented with two ‘TAB’ characters. The above output means that
there is one realm ‘latte’ with two principals:
‘krbtgt/latte’ (which is used to authenticate Kerberos ticket
requests) and ‘host/latte’ (used to authenticate host-based
applications like Telnet). They were created during ‘make
install’ on a host called ‘latte’.

If the installation did not create a default database for you, you
might get an error similar to the following output.

This indicates that the database does not exist. For a file database,
you can create it simply by creating the directory, as follows. Note the
access permission change with ‘chmod’. Typically the ‘root’
user would own the files, but as these examples demonstrate, setting
up a Kerberos server does not require root access. Indeed, it may be
prudent to run all Shishi applications as a special non-‘root’
user, and have all Shishi related files owned by that user, so that
any security vulnerabilities do not lead to a system compromise.
(However, if the user database is ever stolen, system compromises of other
systems may be inoccured, should you use, e.g., a kerberized Telnet.)

Back to the first example, where you have a realm ‘latte’ with
some principals. We want to remove the realm to demonstrate how you
create the realm from scratch. (Of course, you can have more than one
realm in the database, but for this example we assume you want to set
up a realm named the same as Shishi guessed you would name it, so the
existing realm need to be removed first.) The ‘--remove’ (short
form ‘-r’) parameter is used for this purpose, as follows.

You may be asking yourself “What if the realm has many more
principals?”. If you fear manual labor (or a small ‘sed’
script, recall the format of ‘--list’?), don't worry, there is a
‘--force’ (short form ‘-f’) flag. Use it with care. Here is a
faster way to do the above:

You should now have a working, but empty, Shisa database. Let's set
up the realm manually, step by step. The first step is to decide on
a name for your realm. The full story is explained elsewhere
(see Realm and Principal Naming), but the short story is to take
your DNS domain name and translate it to upper case. For
example, if your organization uses example.org it is a good
idea to use EXAMPLE.ORG as the name of your Kerberos realm.
We'll use EXAMPLE.ORG as the realm name in these examples.
Let's create the realm.

Currently, there are no properties associated with entire realms. In
the future, it may be possible to set a default realm-wide password
expiry policy or similar. Each realm normally has one principal that
is used for authenticating against the “ticket granting service” on
the Kerberos server with a ticket instead of using the password. This
is used by the user when she acquire a ticket for a server. The
principal must look like ‘krbtgt/REALM’ (see Name of the TGS). Let's create it.

Now that wasn't difficult, although not very satisfying either. What
does adding a principal mean? The name is created, obviously, but it
also means setting a few values in the database. Let's view the entry
to find out which values.

To use host based security services like SSH or Telnet with
Kerberos, each host must have a key shared between the host and the
KDC. The key is typically stored in
/usr/local/etc/shishi/shishi.keys. We assume your server is
called ‘mail.example.org’ and we create the principal. To
illustrate a new parameter, we also set the specific algorithm to use
by using the ‘--encryption-type’ (short form ‘-E’)
parameter.

The next logical step is to create a principal for some user, so you
can use your password to get a Ticket Granting Ticket via the
Authentication Service (AS) from the KDC, and then use the Ticket
Granting Service (TGS) from the KDC to get a ticket for a specific
host, and then send that ticket to the host to authenticate yourself.
Creating this end-user principle is slightly different from the
earlier steps, because you want the key to be derived from a password
instead of being a random key. The ‘--password’ parameter
indicate this. This make the tool ask you for the password.

3.4 Starting Shishid

The Shishi server, or Key Distribution Center (KDC), is called
Shishid. Shishid is responsible for listening on UDP and TCP ports
for Kerberos requests. Currently it can handle initial ticket
requests (Authentication Service, or AS), typically authenticated with
keys derived from passwords, and subsequent ticket requests (Ticket
Granting Service, or TGS), typically authenticated with the key
acquired during an AS exchange.

Currently there is very little configuration available, the only
variables are which ports the server should listen on and an optional
user name to setuid into after successfully listening to the
ports.

By default, Shishid listens on the ‘kerberos’ service port
(typically translated to 88 via /etc/services) on the UDP and
TCP transports via IPv4 and (if your machine support it) IPv6 on all
interfaces on your machine. Here is a typical startup.

latte:/home/jas/src/shishi# /usr/local/sbin/shishid
Initializing GNUTLS...
Initializing GNUTLS...done
Listening on IPv4:*:kerberos/udp...done
Listening on IPv4:*:kerberos/tcp...done
Listening on IPv6:*:kerberos/udp...failed
socket: Address family not supported by protocol
Listening on IPv6:*:kerberos/tcp...failed
socket: Address family not supported by protocol
Listening on 2 ports...

Running as root is not recommended. Any security problem in shishid
and your host may be compromised. Therefor, we recommend using the
‘--setuid’ parameter, as follows.

latte:/home/jas/src/shishi# /usr/local/sbin/shishid --setuid=jas
Initializing GNUTLS...
Initializing GNUTLS...done
Listening on IPv4:*:kerberos/udp...done
Listening on IPv4:*:kerberos/tcp...done
Listening on IPv6:*:kerberos/udp...failed
socket: Address family not supported by protocol
Listening on IPv6:*:kerberos/tcp...failed
socket: Address family not supported by protocol
Listening on 2 ports...
User identity set to `jas' (22541)...

An alternative is to run shishid on an alternative port as a
non-privileged user. To continue the example of setting up the
EXAMPLE.ORG realm as a non-privileged user from the preceding
section, we start the server listen on port 4711 via UDP on IPv4.

You may use the '-v' parameter for Shishid and Shishi to generate more
debugging information.

To illustrate what an application, such as the Shishi patched versions
of GNU lsh or Telnet from GNU InetUtils, would do
when contacting the host ‘mail.example.org’ we illustrate using
the TGS service as well.

This conclude our walk-through of setting up a new Kerberos realm
using Shishi. It is quite likely that one or more steps failed, and
if so we encourage you to debug it and submit a patch, or at least
report it as a problem. Heck, even letting us know if you got this
far would be of interest. See Bug Reports.

3.5 Configuring DNS for KDC

Making sure the configuration files on all hosts running Shishi
clients include the addresses of your server is tedious. If the
configuration files do not mention the KDC address for a realm, Shishi
will try to look up the information from DNS. In order for Shishi to
find that information, you need to add the information to DNS. For
this to work well, you need to set up a DNS zone with the same name as
your Kerberos realm. The easiest is if you own the publicly visible
DNS name, such as ‘example.org’ if your realm is
‘EXAMPLE.ORG’, but you can set up an internal DNS server with the
information for your realm only. If this is done, you do not need to
keep configuration files updated for the KDC addressing information.

3.5.1 DNS vs. Kerberos - Case Sensitivity of Realm Names

In Kerberos, realm names are case sensitive. While it is strongly
encouraged that all realm names be all upper case this recommendation
has not been adopted by all sites. Some sites use all lower case
names and other use mixed case. DNS on the other hand is case
insensitive for queries but is case preserving for responses to TXT
queries. Since "MYREALM", "myrealm", and "MyRealm" are all different
it is necessary that only one of the possible combinations of upper
and lower case characters be used. This restriction may be lifted in
the future as the DNS naming scheme is expanded to support non-ASCII
names.

3.5.2 Overview - KDC location information

KDC location information is to be stored using the DNS SRV RR [RFC
2052]. The format of this RR is as follows:

Service.Proto.Realm TTL Class SRV Priority Weight Port Target

The Service name for Kerberos is always "_kerberos".

The Proto can be either "_udp", "_tcp", or "_tls._tcp". If these SRV
records are to be used, a "_udp" record MUST be included. If the
Kerberos implementation supports TCP transport, a "_tcp" record MUST
be included. When using "_tcp" with "_kerberos", this indicates a
"raw" TCP connection without any additional encapsulation. A
"_tls._tcp" record MUST be specified for all Kerberos implementations
that support communication with the KDC across TCP sockets
encapsulated using TLS [RFC2246] (see STARTTLS protected KDC exchanges).

The Realm is the Kerberos realm that this record corresponds to.

TTL, Class, SRV, Priority, Weight, and Target have the standard
meaning as defined in RFC 2052.

As per RFC 2052 the Port number should be the value assigned to
"kerberos" by the Internet Assigned Number Authority (88).

3.5.3 Example - KDC location information

These are DNS records for a Kerberos realm ASDF.COM. It has two
Kerberos servers, kdc1.asdf.com and kdc2.asdf.com. Queries should be
directed to kdc1.asdf.com first as per the specified priority.
Weights are not used in these records.

3.5.4 Security considerations

As DNS is deployed today, it is an unsecure service. Thus the infor-
mation returned by it cannot be trusted.

Current practice for REALM to KDC mapping is to use hostnames to
indicate KDC hosts (stored in some implementation-dependent location,
but generally a local config file). These hostnames are vulnerable
to the standard set of DNS attacks (denial of service, spoofed
entries, etc). The design of the Kerberos protocol limits attacks of
this sort to denial of service. However, the use of SRV records does
not change this attack in any way. They have the same vulnerabilities
that already exist in the common practice of using hostnames for
KDC locations.

Implementations SHOULD provide a way of specifying this information
locally without the use of DNS. However, to make this feature
worthwhile a lack of any configuration information on a client should
be interpretted as permission to use DNS.

3.6 Kerberos via TLS

If Shishi is built with support for GNUTLS, the messages exchanged
between clients and Shishid can be protected with TLS. TLS is only
available over TCP connections. A full discussion of the features TLS
have is out of scope here, but in short it means the communication is
integrity and privacy protected, and that users can use OpenPGP, X.509
or SRP (i.e., any mechanism supported by TLS) to authenticate
themselves to the Kerberos server. For details on the implementation,
See STARTTLS protected KDC exchanges.

3.6.1 Setting up TLS resume

Resuming earlier TLS session is supported and enabled by default.
This improves the speed of the TLS handshake, because results from
earlier negotiations can be re-used. Currently the TLS resume
database is stored in memory (in constract to storing it on disk), in
both the client and in the server. Because the server typically runs
for a long time, this is not a problem for that side. The client is
typically not a long-running process though; the client usually is
invoked as part of applications like ‘telnet’ or ‘login’.
However, because each use of the client library typically result in a
ticket, which is stored on disk and re-used by later processes, this
is likely not a serious problem because the number of different
tickets required by a user is usually quite small. For the client,
TLS resume is typically only useful when you perform an initial
authentication (using a password) followed by a ticket request for a
service, in the same process.

You can configure the server, ‘shishid’ to never use TLS resume,
or to increase or decrease the number of distinct TLS connections that
can be resumed before they are garbage collected, see the
‘--resume-limit’ parameter (see Parameters for shishid).

3.6.2 Setting up Anonymous TLS

Anonymous TLS is the simplest to set up and use. In fact, only the
client need to be informed that your KDC support TLS. This can be
done in the configuration file with the ‘/tls’ parameter for
‘kdc-realm’ (see Shishi Configuration), or by
placing the KDC address in DNS using the ‘_tls’ SRV record
(see Configuring DNS for KDC).

3.6.3 Setting up X.509 authenticated TLS

Setting up X.509 authentication is slightly more complicated than
anonymous authentication. You need a X.509 certificate authority
(CA) that can generate certificates for your Kerberos server
and Kerberos clients. It is often easiest to setup the CA
yourself. Managing a CA can be a daunting task, and we only
give the bare essentials to get things up and running. We suggest
that you study the relevant literature. As a first step beyond this
introduction, you may wish to explore more secure forms of key storage
than storing them unencrypted on disk.

The following three sections describe how you create the CA,
KDC certificate, and client certificates. You can use any tool you
like for this task, as long as they generate X.509 (PKIX) certificates
in PEM format and RSA keys in PKCS#1 format. Here we use
certtool that come with GNUTLS, which is widely
available. We conclude by discussing how you use these certificates
in the KDC and in the Shishi client.

3.6.3.4 Starting KDC with X.509 authentication support

The KDC need the CA certificate (to verify client
certificates) and the server certificate and key (to authenticate
itself to the clients). See elsewhere (see Parameters for shishid) for the entire description of the parameters.

Then acquire tickets as usual. In case you wonder how shishi finds
the client certificate and key, the filenames used above when
generating the client certificates happen to be the default filenames
for these files. So it pick them up automatically.

3.7 Multiple servers

Setting up multiple servers is as easy as replicating the user
database. Since the default ‘file’ user database is stored in
the normal file system, you can use any common tools to replicate a
file system. Network file system like NFS (properly secured
by, e.g., a point-to-point symmetrically encrypted IPSEC
connection) and file synchronizing tools like ‘rsync’ are typical
choices.

The secondary server should be configured just like the master server.
If you use the ‘file’ database over NFS you do not have
to make any modifications. If you use, e.g., a cron job to
‘rsync’ the directory every hour or so, you may want to add a
‘--read-only’ flag to the Shisa ‘db’ definition
(see Shisa Configuration). That way, nobody will be lured into
creating or changing information in the database on the secondary
server, which only would be overwritten during the next
synchronization.

db --read-only file /usr/local/var/backup-shishi

The ‘file’ database is designed so it doesn't require file
locking in the file system, which may be unreliable in some network
file systems or implementations. It is also designed so that multiple
concurrent readers and writers may access the database without causing
corruption.

Warning: The last paragraph is currently not completely
accurate. There may be race conditions with concurrent writers. None
should cause infinite loops or data loss. However, unexpected results
might occur if two writers try to update information about a principal
simultaneous.

If you use a remote LDAP server or SQL database to
store the user database, and access it via a Shisa backend, you have
make sure your Shisa backend handle concurrent writers properly. If
you use a modern SQL database, this probably is not a
concern. If it is a problem, you may be able to work around it by
implementing some kind of synchronization or semaphore mechanism. If
all else sounds too complicated, you can set up the secondary servers
as ‘--read-only’ servers, although you will lose some
functionality (like changing passwords via the secondary server, or
updating timestamps when the last ticket request occurred).

One function that is of particular use for users with remote databases
(be it LDAP or SQL) is the “database override”
feature. Using this you can have the security critical principals
(such as the ticket granting ticket) stored on local file system
storage, but use the remote database for user principals. Of course,
you must keep the local file system storage synchronized between all
servers, as before. Here is an example configuration.

This instruct the Shisa library to access the two databases
sequentially, for each query using the first database that know about
the requested principal. If you put the ‘krbtgt/REALM’ principal
in the local ‘file’ database, this will override the
LDAP interface. Naturally, you can have as many ‘db’
definition lines as you wish.

Users with remote databases can also investigate a so called High
Availability mode. This is useful if you wish to have your Kerberos
servers be able to continue to operate even when the remote database
is offline. This is achieved via the ‘--ignore-errors’ flag in
the database definition. Here is a sample configuration.

This instruct the Shisa library to try the LDAP backend
first, but if it fails, instead of returning an error, continue to try
the operation on a read only local ‘file’ based database. Of
course, write requests will still fail, but it may be better than
halting the server completely. To make this work, you first need to
set up a cron job on a, say, hourly basis, to make a copy of the
remote database and store it in the local file database. That way,
when the remote server goes away, fairly current information will
still be available locally.

If you also wish to experiment with read-write fail over, here is an
idea for the configuration.

This is similar to the previous, but it will ignore errors reading and
writing from the first two databases, ultimately causing write
attempts to end up in the final ‘file’ based database. Of
course, you would need to create tools to feed back any local updates
made while the remote server was down. It may also be necessary to
create a special backend for this purpose, which can auto create
principals that are used.

3.8 Developer information

The Programming API for Shisa is described below (see Kerberos Database Functions); this section is about extending Shisa, and
consequently Shishi, to use your own user database system. You may
want to store your Kerberos user information on an LDAP database
server, for example.

Adding a new backend is straight forward. You need to implement the
backend API function set, add the list of API functions to
db/db.c and possibly also add any library dependencies to the
Makefile.

The simplest way to write a new backend is to start from the existing
‘file’ based database, in db/file.c, and modify the entry
points as needed.

Note that the current backend API will likely change before it is
frozen. We may describe it in detail here when it has matured.
However, currently it is similar to the external Shisa API
(see Kerberos Database Functions).

There should be no need to modify anything else in the Shisa library,
and certainly not in the Shishi library or the shishid server.

Naturally, we would appreciate if you would send us your new backend,
if you believe it is generally useful (see Bug Reports).

4 Reference Manual

This chapter discuss the underlying assumptions of Kerberos, contain a
glossary to Kerberos concepts, give you background information on
choosing realm and principal names, and describe all parameters and
configuration file syntaxes for the Shishi tools.

4.1 Environmental Assumptions

Kerberos imposes a few assumptions on the environment in which it can
properly function:

"Denial of service" attacks are not solved with Kerberos. There
are places in the protocols where an intruder can prevent an
application from participating in the proper authentication steps.
Detection and solution of such attacks (some of which can appear
to be not-uncommon "normal" failure modes for the system) is
usually best left to the human administrators and users.

Principals MUST keep their secret keys secret. If an intruder
somehow steals a principal's key, it will be able to masquerade as
that principal or impersonate any server to the legitimate
principal.

"Password guessing" attacks are not solved by Kerberos. If a user
chooses a poor password, it is possible for an attacker to
successfully mount an offline dictionary attack by repeatedly
attempting to decrypt, with successive entries from a dictionary,
messages obtained which are encrypted under a key derived from the
user's password.

Each host on the network MUST have a clock which is "loosely
synchronized" to the time of the other hosts; this synchronization
is used to reduce the bookkeeping needs of application servers
when they do replay detection. The degree of "looseness" can be
configured on a per-server basis, but is typically on the order of
5 minutes. If the clocks are synchronized over the network, the
clock synchronization protocol MUST itself be secured from network
attackers.

Principal identifiers are not recycled on a short-term basis. A
typical mode of access control will use access control lists
(ACLs) to grant permissions to particular principals. If a stale
ACL entry remains for a deleted principal and the principal
identifier is reused, the new principal will inherit rights
specified in the stale ACL entry. By not re-using principal
identifiers, the danger of inadvertent access is removed.

4.2 Glossary of terms

A record containing a Ticket and an Authenticator to be presented
to a server as part of the authentication process.

Authentication path

A sequence of intermediate realms transited in the authentication
process when communicating from one realm to another.

Authenticator

A record containing information that can be shown to have been
recently generated using the session key known only by the client
and server.

Authorization

The process of determining whether a client may use a service,
which objects the client is allowed to access, and the type of
access allowed for each.

Capability

A token that grants the bearer permission to access an object or
service. In Kerberos, this might be a ticket whose use is
restricted by the contents of the authorization data field, but
which lists no network addresses, together with the session key
necessary to use the ticket.

Ciphertext

The output of an encryption function. Encryption transforms
plaintext into ciphertext.

Client

A process that makes use of a network service on behalf of a user.
Note that in some cases a Server may itself be a client of some
other server (e.g. a print server may be a client of a file
server).

Credentials

A ticket plus the secret session key necessary to successfully use
that ticket in an authentication exchange.

Encryption Type (etype)

When associated with encrypted data, an encryption type identifies
the algorithm used to encrypt the data and is used to select the
appropriate algorithm for decrypting the data. Encryption type
tags are communicated in other messages to enumerate algorithms
that are desired, supported, preferred, or allowed to be used for
encryption of data between parties. This preference is combined
with local information and policy to select an algorithm to be
used.

KDC

Key Distribution Center, a network service that supplies tickets
and temporary session keys; or an instance of that service or the
host on which it runs. The KDC services both initial ticket and
ticket-granting ticket requests. The initial ticket portion is
sometimes referred to as the Authentication Server (or service).
The ticket-granting ticket portion is sometimes referred to as the
ticket-granting server (or service).

Kerberos

The name given to the Project Athena's authentication service, the
protocol used by that service, or the code used to implement the
authentication service. The name is adopted from the three-headed
dog which guards Hades.

Key Version Number (kvno)

A tag associated with encrypted data identifies which key was used
for encryption when a long lived key associated with a principal
changes over time. It is used during the transition to a new key
so that the party decrypting a message can tell whether the data
was encrypted using the old or the new key.

Plaintext

The input to an encryption function or the output of a decryption
function. Decryption transforms ciphertext into plaintext.

Principal

A named client or server entity that participates in a network
communication, with one name that is considered canonical.

Principal identifier

The canonical name used to uniquely identify each different
principal.

Seal

To encipher a record containing several fields in such a way that
the fields cannot be individually replaced without either
knowledge of the encryption key or leaving evidence of tampering.

Secret key

An encryption key shared by a principal and the KDC, distributed
outside the bounds of the system, with a long lifetime. In the
case of a human user's principal, the secret key MAY be derived
from a password.

Server

A particular Principal which provides a resource to network
clients. The server is sometimes referred to as the Application
Server.

Service

A resource provided to network clients; often provided by more
than one server (for example, remote file service).

Session key

A temporary encryption key used between two principals, with a
lifetime limited to the duration of a single login "session". In
the Kerberos system, a session key is generated by the KDC. The
session key is distinct from the sub-session key, described next..

Sub-session key

A temporary encryption key used between two principals, selected
and exchanged by the principals using the session key, and with a
lifetime limited to the duration of a single association. The sub-
session key is also referred to as the subkey.

Ticket

A record that helps a client authenticate itself to a server; it
contains the client's identity, a session key, a timestamp, and
other information, all sealed using the server's secret key. It
only serves to authenticate a client when presented along with a
fresh Authenticator.

4.3 Realm and Principal Naming

This section contains the discussion on naming realms and principals
from the Kerberos specification.

4.3.1 Realm Names

Although realm names are encoded as GeneralStrings and although a
realm can technically select any name it chooses, interoperability
across realm boundaries requires agreement on how realm names are to
be assigned, and what information they imply.

To enforce these conventions, each realm MUST conform to the
conventions itself, and it MUST require that any realms with which
inter-realm keys are shared also conform to the conventions and
require the same from its neighbors.

Kerberos realm names are case sensitive. Realm names that differ only
in the case of the characters are not equivalent. There are presently
three styles of realm names: domain, X500, and other. Examples of
each style follow:

Domain syle realm names MUST look like domain names: they consist of
components separated by periods (.) and they contain neither colons
(:) nor slashes (/). Though domain names themselves are case
insensitive, in order for realms to match, the case must match as
well. When establishing a new realm name based on an internet domain
name it is recommended by convention that the characters be converted
to upper case.

X.500 names contain an equal (=) and cannot contain a colon (:)
before the equal. The realm names for X.500 names will be string
representations of the names with components separated by slashes.
Leading and trailing slashes will not be included. Note that the
slash separator is consistent with Kerberos implementations based on
RFC1510, but it is different from the separator recommended in
RFC2253.

Names that fall into the other category MUST begin with a prefix that
contains no equal (=) or period (.) and the prefix MUST be followed
by a colon (:) and the rest of the name. All prefixes must be
assigned before they may be used. Presently none are assigned.

The reserved category includes strings which do not fall into the
first three categories. All names in this category are reserved. It
is unlikely that names will be assigned to this category unless there
is a very strong argument for not using the 'other' category.

These rules guarantee that there will be no conflicts between the
various name styles. The following additional constraints apply to
the assignment of realm names in the domain and X.500 categories: the
name of a realm for the domain or X.500 formats must either be used
by the organization owning (to whom it was assigned) an Internet
domain name or X.500 name, or in the case that no such names are
registered, authority to use a realm name MAY be derived from the
authority of the parent realm. For example, if there is no domain
name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can
authorize the creation of a realm with that name.

This is acceptable because the organization to which the parent is
assigned is presumably the organization authorized to assign names to
its children in the X.500 and domain name systems as well. If the
parent assigns a realm name without also registering it in the domain
name or X.500 hierarchy, it is the parent's responsibility to make
sure that there will not in the future exist a name identical to the
realm name of the child unless it is assigned to the same entity as
the realm name.

4.3.2 Principal Names

As was the case for realm names, conventions are needed to ensure
that all agree on what information is implied by a principal name.
The name-type field that is part of the principal name indicates the
kind of information implied by the name. The name-type SHOULD be
treated only as a hint to interpreting the meaning of a name. It is
not significant when checking for equivalence. Principal names that
differ only in the name-type identify the same principal. The name
type does not partition the name space. Ignoring the name type, no
two names can be the same (i.e. at least one of the components, or
the realm, MUST be different). The following name types are defined:

name-type value meaning
NT-UNKNOWN 0 Name type not known
NT-PRINCIPAL 1 Just the name of the principal as in DCE, or for users
NT-SRV-INST 2 Service and other unique instance (krbtgt)
NT-SRV-HST 3 Service with host name as instance (telnet, rcommands)
NT-SRV-XHST 4 Service with host as remaining components
NT-UID 5 Unique ID
NT-X500-PRINCIPAL 6 Encoded X.509 Distingished name [RFC 2253]
NT-SMTP-NAME 7 Name in form of SMTP email name (e.g. user@foo.com)
NT-ENTERPRISE 10 Enterprise name - may be mapped to principal name

When a name implies no information other than its uniqueness at a
particular time the name type PRINCIPAL SHOULD be used. The principal
name type SHOULD be used for users, and it might also be used for a
unique server. If the name is a unique machine generated ID that is
guaranteed never to be reassigned then the name type of UID SHOULD be
used (note that it is generally a bad idea to reassign names of any
type since stale entries might remain in access control lists).

If the first component of a name identifies a service and the
remaining components identify an instance of the service in a server
specified manner, then the name type of SRV-INST SHOULD be used. An
example of this name type is the Kerberos ticket-granting service
whose name has a first component of krbtgt and a second component
identifying the realm for which the ticket is valid.

If the first component of a name identifies a service and there is a
single component following the service name identifying the instance
as the host on which the server is running, then the name type SRV-
HST SHOULD be used. This type is typically used for Internet services
such as telnet and the Berkeley R commands. If the separate
components of the host name appear as successive components following
the name of the service, then the name type SRV-XHST SHOULD be used.
This type might be used to identify servers on hosts with X.500 names
where the slash (/) might otherwise be ambiguous.

A name type of NT-X500-PRINCIPAL SHOULD be used when a name from an
X.509 certificate is translated into a Kerberos name. The encoding of
the X.509 name as a Kerberos principal shall conform to the encoding
rules specified in RFC 2253.

A name type of SMTP allows a name to be of a form that resembles a
SMTP email name. This name, including an "@" and a domain name, is
used as the one component of the principal name.

A name type of UNKNOWN SHOULD be used when the form of the name is
not known. When comparing names, a name of type UNKNOWN will match
principals authenticated with names of any type. A principal
authenticated with a name of type UNKNOWN, however, will only match
other names of type UNKNOWN.

Names of any type with an initial component of 'krbtgt' are reserved
for the Kerberos ticket granting service. See Name of the TGS, for the form of such names.

4.3.2.1 Name of server principals

The principal identifier for a server on a host will generally be
composed of two parts: (1) the realm of the KDC with which the server
is registered, and (2) a two-component name of type NT-SRV-HST if the
host name is an Internet domain name or a multi-component name of
type NT-SRV-XHST if the name of the host is of a form such as X.500
that allows slash (/) separators. The first component of the two- or
multi-component name will identify the service and the latter
components will identify the host. Where the name of the host is not
case sensitive (for example, with Internet domain names) the name of
the host MUST be lower case. If specified by the application protocol
for services such as telnet and the Berkeley R commands which run
with system privileges, the first component MAY be the string 'host'
instead of a service specific identifier.

4.3.2.2 Name of the TGS

The principal identifier of the ticket-granting service shall be
composed of three parts: (1) the realm of the KDC issuing the TGS
ticket (2) a two-part name of type NT-SRV-INST, with the first part
"krbtgt" and the second part the name of the realm which will accept
the ticket-granting ticket. For example, a ticket-granting ticket
issued by the ATHENA.MIT.EDU realm to be used to get tickets from the
ATHENA.MIT.EDU KDC has a principal identifier of "ATHENA.MIT.EDU"
(realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A ticket-granting
ticket issued by the ATHENA.MIT.EDU realm to be used to get tickets
from the MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU"
(realm), ("krbtgt", "MIT.EDU") (name).

4.3.3 Choosing a principal with which to communicate

The Kerberos protocol provides the means for verifying (subject to
the assumptions in Environmental Assumptions) that the entity with which one communicates
is the same entity that was registered with the KDC using the claimed
identity (principal name). It is still necessary to determine whether
that identity corresponds to the entity with which one intends to
communicate.

When appropriate data has been exchanged in advance, this
determination may be performed syntactically by the application based
on the application protocol specification, information provided by
the user, and configuration files. For example, the server principal
name (including realm) for a telnet server might be derived from the
user specified host name (from the telnet command line), the "host/"
prefix specified in the application protocol specification, and a
mapping to a Kerberos realm derived syntactically from the domain
part of the specified hostname and information from the local
Kerberos realms database.

One can also rely on trusted third parties to make this
determination, but only when the data obtained from the third party
is suitably integrity protected while resident on the third party
server and when transmitted. Thus, for example, one should not rely
on an unprotected domain name system record to map a host alias to
the primary name of a server, accepting the primary name as the party
one intends to contact, since an attacker can modify the mapping and
impersonate the party with which one intended to communicate.

Implementations of Kerberos and protocols based on Kerberos MUST NOT
use insecure DNS queries to canonicalize the hostname components of
the service principal names. In an environment without secure name
service, application authors MAY append a statically configured
domain name to unqualified hostnames before passing the name to the
security mechanisms, but should do no more than that. Secure name
service facilities, if available, might be trusted for hostname
canonicalization, but such canonicalization by the client SHOULD NOT
be required by KDC implementations.

Implementation note: Many current implementations do some degree of
canonicalization of the provided service name, often using DNS even
though it creates security problems. However there is no consistency
among implementations about whether the service name is case folded
to lower case or whether reverse resolution is used. To maximize
interoperability and security, applications SHOULD provide security
mechanisms with names which result from folding the user-entered name
to lower case, without performing any other modifications or
canonicalization.

4.3.4 Principal Name Form

Principal names consist of a sequence of strings, which is often
tedious to parse. Therefor, Shishi often uses a “printed” form of
principal which embed the entire principal name string sequence, and
optionally also the realm, into one string. The format is taken from
the Kerberos 5 GSS-API mechanism (RFC 1964).

The elements included within this name representation are as follows,
proceeding from the beginning of the string:

One or more principal name components; if more than one
principal name component is included, the components are
separated by `/`. Arbitrary octets may be included within
principal name components, with the following constraints and
special considerations:

Any occurrence of the characters `@` or `/` within a
name component must be immediately preceded by the `\`
quoting character, to prevent interpretation as a component
or realm separator.

The ASCII newline, tab, backspace, and null characters
may occur directly within the component or may be
represented, respectively, by `\n`, `\t`, `\b`, or `\0`.

If the `\` quoting character occurs outside the contexts
described in (1a) and (1b) above, the following character is
interpreted literally. As a special case, this allows the
doubled representation `\\` to represent a single occurrence
of the quoting character.

An occurrence of the `\` quoting character as the last
character of a component is illegal.

Optionally, a `@` character, signifying that a realm name
immediately follows. If no realm name element is included, the
local realm name is assumed. The `/` , `:`, and null characters
may not occur within a realm name; the `@`, newline, tab, and
backspace characters may be included using the quoting
conventions described in (1a), (1b), and (1c) above.

4.4 Shishi Configuration

The valid configuration file tokens are described here. The user
configuration file is typically located in
~/.shishi/shishi.conf (compare ‘shishi
--configuration-file’) and the system configuration is typically
located in /usr/local/etc/shishi/shishi.conf (compare
‘shishi --system-configuration-file’). If the first non white
space character of a line is a '#', the line is ignored. Empty lines
are also ignored.

All tokens are valid in both the system and the user configuration
files, and have the same meaning. However, as the system file is
supposed to apply to all users on a system, it would not make sense to
use some tokens in that file. For example, the
‘default-principal’ is rarely useful in a system configuration
file.

4.4.1 ‘default-realm’

Specify the default realm, by default the hostname of the host is
used. E.g.,

default-realm JOSEFSSON.ORG

4.4.2 ‘default-principal’

Specify the default principal, by default the login username is
used. E.g.,

default-principal jas

4.4.3 ‘client-kdc-etypes’

Specify which encryption types client asks server to respond in during
AS/TGS exchanges. List valid encryption types, in preference order.
Supported algorithms include aes256-cts-hmac-sha1-96,
aes128-cts-hmac-sha1-96, des3-cbc-sha1-kd, des-cbc-md5, des-cbc-md4,
des-cbc-crc and null. This option also indicates which encryption
types are accepted by the client when receiving the response. Note
that the preference order is not cryptographically protected, so a man
in the middle can modify the order without being detected. Thus, only
specify encryption types you trust completely here. The default only
includes aes256-cts-hmac-sha1-96, as suggested by RFC1510bis. E.g.,

It can also be a string indicating a character set supported by
iconv via libstringprep, in which case data is converted from locale
charset into the indicated character set. E.g., UTF-8, ISO-8859-1,
KOI-8, EBCDIC-IS-FRISS are supported on GNU systems. On some systems
you can use "locale -m" to list available character sets. By default,
the "none" setting is used which is consistent with RFC 1510 that is
silent on the issue. In practice, however, converting to UTF-8
improves interoperability.

E.g.,

stringprocess=UTF-8

4.4.9 ‘ticket-life’

Specify default ticket life time.

The string can be in almost any common format. It can contain month
names, time zones, `am' and `pm', `yesterday', `ago', `next', etc.
See Date input formats, for the long story.

As an extra feature, if the time specified by your string correspond
to a time during the last 24 hours, an extra day is added to it. This
allows you to specify relative times such as "17:00" to always mean
the next 17:00, even if your system clock happens to be 17:30.

The default is 8 hours.

E.g.,

#ticket-life=8 hours
#ticket-life=1 day
ticket-life=17:00

4.4.10 ‘renew-life’

Specify how long a renewable ticket should remain renewable.

See ticket-life for the syntax. The extra feature that handles
negative values within the last 2 hours is not active here.

4.5 Shisa Configuration

The configuration file for Shisa is typically stored in
/usr/local/etc/shishi/shisa.conf. If the first non white space
character of a line is a '#', the line is ignored. Empty lines are
also ignored.

4.5.1 ‘db’

Currently the only configuration options available is the db
token that define the databases to use. The syntax is:

db [OPTIONS] <TYPE> [LOCATION] [PARAMETERS ...]

Specify the data sources for Kerberos 5 data. Multiple entries,
even of the same data source type, are allowed. The data sources
are accessed in the same sequence as they are defined here. If an
entry is found in one data source, it will be used for the
operations, without searching the remaining data sources. Valid
OPTIONS include:

--read-only No data is written to this data source.
--ignore-errors Ignore failures in this backend.

The default (when the configuration file is empty) uses one "file"
data source (see below), but for a larger installation you may want to
combine several data sources. Here is an example.

This demonstrate how you can store critical principals on local disk
(the first entry, /var/local/master) that will always be found without
looking in the LDAP directory. The critical principals could be,
e.g., krbtgt/EXAMPLE.ORG. The second entry denote a LDAP server that
could hold user principals. As you can see, Shisa will not let the
caller know about errors with the LDAP source (they will be logged,
however). Instead, if for instance the LDAP server has crashed, Shisa
would continue and read from the /var/cache/ldap-copy file source.
That file source may have been set up to contain a copy of the data in
the LDAP server, perhaps made on an hourly basis, so that your server
will be able to serve recent data even in case of a crash. Any
updates or passwords change requests will however not be possible
while the LDAP server is inaccessible, to reduce the problem of
synchronizing data back into the LDAP server once it is online again.

Currently only the "file" data source is supported, and denote a
data source that use the standard file system for storage.

4.6 Parameters for shishi

If no command is given, Shishi try to make sure you have a ticket
granting ticket for the default realm, and then display it.

Mandatory arguments to long options are mandatory for short options
too.

Usage: shishi [OPTIONS]... [CLIENT [SERVER]]...
-h, --help Print help and exit
-V, --version Print version and exit
Commands:
-d, --destroy Destroy tickets in local cache,
limited by any --client-name or
--server-name. (default=off)
-l, --list List tickets in local cache, limited
by any --client-name and
--server-name. (default=off)
-r, --renew Renew ticket. Use --server-name to
specify ticket, default is the
most recent renewable ticket
granting ticket for the default
realm. (default=off)
Flags:
--forwardable Get a forwardable ticket, i.e., one
that can be used to get forwarded
tickets. (default=off)
--forwarded Get a forwarded ticket. (default=
off)
--proxiable Get a proxiable ticket, i.e., one
that can be used to get proxy
tickets. (default=off)
--proxy Get a proxy ticket. (default=off)
--renewable Get a renewable ticket. (default=
off)
Options:
--client-name=NAME Client name. Default is login
username.
-E, --encryption-type=ETYPE,[ETYPE...] Encryption types to use. ETYPE is
either registered name or integer.
Valid values include 'aes128',
'aes256', 'aes' (same as
'aes256'), '3des', 'des-md5',
'des-md4', 'des-crc', 'des' (same
as 'des-md5'), and 'arcfour'.
-e, --endtime=STRING Specify when ticket validity should
expire. The time syntax may be
relative (to the start time), such
as '20 hours', or absolute, such
as '2001-02-03 04:05:06 CET'. The
default is 8 hours after the start
time.
--realm=STRING Set default realm.
--renew-till=STRING Specify renewable life of ticket.
Implies --renewable. Accepts same
time syntax as --endtime. If
--renewable is specified, the
default is 1 week after the start
time.
--server-name=NAME Server name. Default is
'krbtgt/REALM' where REALM is
client realm.
-s, --starttime=STRING Specify when ticket should start to
be valid. Accepts same time
syntax as --endtime. The default
is to become valid immediately.
--ticket-granter=NAME Service name in ticket to use for
authenticating request. Only for
TGS. Defaults to
'krbtgt/REALM@REALM' where REALM
is client realm.
Other options:
--configuration-file=FILE Read user configuration from FILE.
-c, --ticket-file=FILE Read tickets from FILE.
-o, --library-options=STRING Parse STRING as a configuration file
statement.
-q, --quiet Don't produce any diagnostic output.
(default=off)
--system-configuration-file=FILE Read system configuration from FILE.
--ticket-write-file=FILE Write tickets from FILE. Default is
to write them back to where they
were read from.
-v, --verbose Produce verbose output.
(default=off)

4.9 Environment variables

A few of the compile-time defaults may be overridden at run-time by
using environment variables. The following variables are supported.

SHISHI_CONFIG
Specify the location of the default system configuration file. Used
by the Shishi library. If not specified, the default is specified at
compile-time and is usually $prefix/etc/shishi.conf.

SHISHI_HOME
Specify the user specific directory for configuration files, ticket
cache, etc. Used by the Shishi library. If not specified, it is
computed as $HOME/.shishi.

SHISHI_USER
Specify the default principal user name. Used by the Shishi library.
If not specified, it is taken from the environment variable
USER.

SHISHI_TICKETS
Specify the file name of the ticket cache. Used by the Shishi
library. If not specified, it will be $SHISHI_HOME/tickets, or
$HOME/.shishi/tickets if $SHISHI_HOME is not specified.

4.10 Date input formats

First, a quote:

Our units of temporal measurement, from seconds on up to months, are so
complicated, asymmetrical and disjunctive so as to make coherent mental
reckoning in time all but impossible. Indeed, had some tyrannical god
contrived to enslave our minds to time, to make it all but impossible
for us to escape subjection to sodden routines and unpleasant surprises,
he could hardly have done better than handing down our present system.
It is like a set of trapezoidal building blocks, with no vertical or
horizontal surfaces, like a language in which the simplest thought
demands ornate constructions, useless particles and lengthy
circumlocutions. Unlike the more successful patterns of language and
science, which enable us to face experience boldly or at least
level-headedly, our system of temporal calculation silently and
persistently encourages our terror of time.
...

It is as though architects had to measure length in feet, width
in meters and height in ells; as though basic instruction manuals
demanded a knowledge of five different languages. It is no wonder then
that we often look into our own immediate past or future, last Tuesday
or a week from Sunday, with feelings of helpless confusion. ...

—Robert Grudin, Time and the Art of Living.

This section describes the textual date representations that GNU
programs accept. These are the strings you, as a user, can supply as
arguments to the various programs. The C interface (via the
parse_datetime function) is not described here.

4.10.1 General date syntax

A date is a string, possibly empty, containing many items
separated by whitespace. The whitespace may be omitted when no
ambiguity arises. The empty string means the beginning of today (i.e.,
midnight). Order of the items is immaterial. A date string may contain
many flavors of items:

calendar date items

time of day items

time zone items

combined date and time of day items

day of the week items

relative items

pure numbers.

We describe each of these item types in turn, below.

A few ordinal numbers may be written out in words in some contexts. This is
most useful for specifying day of the week items or relative items (see
below). Among the most commonly used ordinal numbers, the word
‘last’ stands for -1, ‘this’ stands for 0, and
‘first’ and ‘next’ both stand for 1. Because the word
‘second’ stands for the unit of time there is no way to write the
ordinal number 2, but for convenience ‘third’ stands for 3,
‘fourth’ for 4, ‘fifth’ for 5,
‘sixth’ for 6, ‘seventh’ for 7, ‘eighth’ for 8,
‘ninth’ for 9, ‘tenth’ for 10, ‘eleventh’ for 11 and
‘twelfth’ for 12.

When a month is written this way, it is still considered to be written
numerically, instead of being “spelled in full”; this changes the
allowed strings.

In the current implementation, only English is supported for words and
abbreviations like ‘AM’, ‘DST’, ‘EST’, ‘first’,
‘January’, ‘Sunday’, ‘tomorrow’, and ‘year’.

The output of the date command
is not always acceptable as a date string,
not only because of the language problem, but also because there is no
standard meaning for time zone items like ‘IST’. When using
date to generate a date string intended to be parsed later,
specify a date format that is independent of language and that does not
use time zone items other than ‘UTC’ and ‘Z’. Here are some
ways to do this:

Alphabetic case is completely ignored in dates. Comments may be introduced
between round parentheses, as long as included parentheses are properly
nested. Hyphens not followed by a digit are currently ignored. Leading
zeros on numbers are ignored.

Invalid dates like ‘2005-02-29’ or times like ‘24:00’ are
rejected. In the typical case of a host that does not support leap
seconds, a time like ‘23:59:60’ is rejected even if it
corresponds to a valid leap second.

The year can also be omitted. In this case, the last specified year is
used, or the current year if none. For example:

9/24
sep 24

Here are the rules.

For numeric months, the ISO 8601 format
‘year-month-day’ is allowed, where year is
any positive number, month is a number between 01 and 12, and
day is a number between 01 and 31. A leading zero must be present
if a number is less than ten. If year is 68 or smaller, then 2000
is added to it; otherwise, if year is less than 100,
then 1900 is added to it. The construct
‘month/day/year’, popular in the United States,
is accepted. Also ‘month/day’, omitting the year.

Literal months may be spelled out in full: ‘January’,
‘February’, ‘March’, ‘April’, ‘May’, ‘June’,
‘July’, ‘August’, ‘September’, ‘October’,
‘November’ or ‘December’. Literal months may be abbreviated
to their first three letters, possibly followed by an abbreviating dot.
It is also permitted to write ‘Sept’ instead of ‘September’.

When months are written literally, the calendar date may be given as any
of the following:

More generally, the time of day may be given as
‘hour:minute:second’, where hour is
a number between 0 and 23, minute is a number between 0 and
59, and second is a number between 0 and 59 possibly followed by
‘.’ or ‘,’ and a fraction containing one or more digits.
Alternatively,
‘:second’ can be omitted, in which case it is taken to
be zero. On the rare hosts that support leap seconds, second
may be 60.

If the time is followed by ‘am’ or ‘pm’ (or ‘a.m.’
or ‘p.m.’), hour is restricted to run from 1 to 12, and
‘:minute’ may be omitted (taken to be zero). ‘am’
indicates the first half of the day, ‘pm’ indicates the second
half of the day. In this notation, 12 is the predecessor of 1:
midnight is ‘12am’ while noon is ‘12pm’.
(This is the zero-oriented interpretation of ‘12am’ and ‘12pm’,
as opposed to the old tradition derived from Latin
which uses ‘12m’ for noon and ‘12pm’ for midnight.)

The time may alternatively be followed by a time zone correction,
expressed as ‘shhmm’, where s is ‘+’
or ‘-’, hh is a number of zone hours and mm is a number
of zone minutes.
The zone minutes term, mm, may be omitted, in which case
the one- or two-digit correction is interpreted as a number of hours.
You can also separate hh from mm with a colon.
When a time zone correction is given this way, it
forces interpretation of the time relative to
Coordinated Universal Time (UTC), overriding any previous
specification for the time zone or the local time zone. For example,
‘+0530’ and ‘+05:30’ both stand for the time zone 5.5 hours
ahead of UTC (e.g., India).
This is the best way to
specify a time zone correction by fractional parts of an hour.
The maximum zone correction is 24 hours.

Either ‘am’/‘pm’ or a time zone correction may be specified,
but not both.

4.10.4 Time zone items

A time zone item specifies an international time zone, indicated
by a small set of letters, e.g., ‘UTC’ or ‘Z’
for Coordinated Universal
Time. Any included periods are ignored. By following a
non-daylight-saving time zone by the string ‘DST’ in a separate
word (that is, separated by some white space), the corresponding
daylight saving time zone may be specified.
Alternatively, a non-daylight-saving time zone can be followed by a
time zone correction, to add the two values. This is normally done
only for ‘UTC’; for example, ‘UTC+05:30’ is equivalent to
‘+05:30’.

Time zone items other than ‘UTC’ and ‘Z’
are obsolescent and are not recommended, because they
are ambiguous; for example, ‘EST’ has a different meaning in
Australia than in the United States. Instead, it's better to use
unambiguous numeric time zone corrections like ‘-0500’, as
described in the previous section.

If neither a time zone item nor a time zone correction is supplied,
time stamps are interpreted using the rules of the default time zone
(see Specifying time zone rules).

4.10.5 Combined date and time of day items

The ISO 8601 date and time of day extended format consists of an ISO
8601 date, a ‘T’ character separator, and an ISO 8601 time of
day. This format is also recognized if the ‘T’ is replaced by a
space.

In this format, the time of day should use 24-hour notation.
Fractional seconds are allowed, with either comma or period preceding
the fraction. ISO 8601 fractional minutes and hours are not
supported. Typically, hosts support nanosecond timestamp resolution;
excess precision is silently discarded.

4.10.6 Day of week items

The explicit mention of a day of the week will forward the date
(only if necessary) to reach that day of the week in the future.

Days of the week may be spelled out in full: ‘Sunday’,
‘Monday’, ‘Tuesday’, ‘Wednesday’, ‘Thursday’,
‘Friday’ or ‘Saturday’. Days may be abbreviated to their
first three letters, optionally followed by a period. The special
abbreviations ‘Tues’ for ‘Tuesday’, ‘Wednes’ for
‘Wednesday’ and ‘Thur’ or ‘Thurs’ for ‘Thursday’ are
also allowed.

A number may precede a day of the week item to move forward
supplementary weeks. It is best used in expression like ‘third
monday’. In this context, ‘last day’ or ‘next
day’ is also acceptable; they move one week before or after
the day that day by itself would represent.

4.10.7 Relative items in date strings

Relative items adjust a date (or the current date if none) forward
or backward. The effects of relative items accumulate. Here are some
examples:

1 year
1 year ago
3 years
2 days

The unit of time displacement may be selected by the string ‘year’
or ‘month’ for moving by whole years or months. These are fuzzy
units, as years and months are not all of equal duration. More precise
units are ‘fortnight’ which is worth 14 days, ‘week’ worth 7
days, ‘day’ worth 24 hours, ‘hour’ worth 60 minutes,
‘minute’ or ‘min’ worth 60 seconds, and ‘second’ or
‘sec’ worth one second. An ‘s’ suffix on these units is
accepted and ignored.

The unit of time may be preceded by a multiplier, given as an optionally
signed number. Unsigned numbers are taken as positively signed. No
number at all implies 1 for a multiplier. Following a relative item by
the string ‘ago’ is equivalent to preceding the unit by a
multiplier with value -1.

The string ‘tomorrow’ is worth one day in the future (equivalent
to ‘day’), the string ‘yesterday’ is worth
one day in the past (equivalent to ‘day ago’).

The strings ‘now’ or ‘today’ are relative items corresponding
to zero-valued time displacement, these strings come from the fact
a zero-valued time displacement represents the current time when not
otherwise changed by previous items. They may be used to stress other
items, like in ‘12:00 today’. The string ‘this’ also has
the meaning of a zero-valued time displacement, but is preferred in
date strings like ‘this thursday’.

When a relative item causes the resulting date to cross a boundary
where the clocks were adjusted, typically for daylight saving time,
the resulting date and time are adjusted accordingly.

The fuzz in units can cause problems with relative items. For
example, ‘2003-07-31 -1 month’ might evaluate to 2003-07-01,
because 2003-06-31 is an invalid date. To determine the previous
month more reliably, you can ask for the month before the 15th of the
current month. For example:

Also, take care when manipulating dates around clock changes such as
daylight saving leaps. In a few cases these have added or subtracted
as much as 24 hours from the clock, so it is often wise to adopt
universal time by setting the TZ environment variable to
‘UTC0’ before embarking on calendrical calculations.

4.10.8 Pure numbers in date strings

The precise interpretation of a pure decimal number depends
on the context in the date string.

If the decimal number is of the form yyyymmdd and no
other calendar date item (see Calendar date items) appears before it
in the date string, then yyyy is read as the year, mm as the
month number and dd as the day of the month, for the specified
calendar date.

If the decimal number is of the form hhmm and no other time
of day item appears before it in the date string, then hh is read
as the hour of the day and mm as the minute of the hour, for the
specified time of day. mm can also be omitted.

If both a calendar date and a time of day appear to the left of a number
in the date string, but no relative item, then the number overrides the
year.

4.10.9 Seconds since the Epoch

If you precede a number with ‘@’, it represents an internal time
stamp as a count of seconds. The number can contain an internal
decimal point (either ‘.’ or ‘,’); any excess precision not
supported by the internal representation is truncated toward minus
infinity. Such a number cannot be combined with any other date
item, as it specifies a complete time stamp.

Internally, computer times are represented as a count of seconds since
an epoch—a well-defined point of time. On GNU and
POSIX systems, the epoch is 1970-01-01 00:00:00 UTC, so
‘@0’ represents this time, ‘@1’ represents 1970-01-01
00:00:01 UTC, and so forth. GNU and most other
POSIX-compliant systems support such times as an extension
to POSIX, using negative counts, so that ‘@-1’
represents 1969-12-31 23:59:59 UTC.

Traditional Unix systems count seconds with 32-bit two's-complement
integers and can represent times from 1901-12-13 20:45:52 through
2038-01-19 03:14:07 UTC. More modern systems use 64-bit counts
of seconds with nanosecond subcounts, and can represent all the times
in the known lifetime of the universe to a resolution of 1 nanosecond.

On most hosts, these counts ignore the presence of leap seconds.
For example, on most hosts ‘@915148799’ represents 1998-12-31
23:59:59 UTC, ‘@915148800’ represents 1999-01-01 00:00:00
UTC, and there is no way to represent the intervening leap second
1998-12-31 23:59:60 UTC.

4.10.10 Specifying time zone rules

Normally, dates are interpreted using the rules of the current time
zone, which in turn are specified by the TZ environment
variable, or by a system default if TZ is not set. To specify a
different set of default time zone rules that apply just to one date,
start the date with a string of the form ‘TZ="rule"’. The
two quote characters (‘"’) must be present in the date, and any
quotes or backslashes within rule must be escaped by a
backslash.

For example, with the GNU date command you can
answer the question “What time is it in New York when a Paris clock
shows 6:30am on October 31, 2004?” by using a date beginning with
‘TZ="Europe/Paris"’ as shown in the following shell transcript:

In this example, the --date operand begins with its own
TZ setting, so the rest of that operand is processed according
to ‘Europe/Paris’ rules, treating the string ‘2004-10-31
06:30’ as if it were in Paris. However, since the output of the
date command is processed according to the overall time zone
rules, it uses New York time. (Paris was normally six hours ahead of
New York in 2004, but this example refers to a brief Halloween period
when the gap was five hours.)

A TZ value is a rule that typically names a location in the
‘tz’ database.
A recent catalog of location names appears in the
TWiki Date and Time Gateway. A few non-GNU hosts require a colon before a
location name in a TZ setting, e.g.,
‘TZ=":America/New_York"’.

The ‘tz’ database includes a wide variety of locations ranging
from ‘Arctic/Longyearbyen’ to ‘Antarctica/South_Pole’, but
if you are at sea and have your own private time zone, or if you are
using a non-GNU host that does not support the ‘tz’
database, you may need to use a POSIX rule instead. Simple
POSIX rules like ‘UTC0’ specify a time zone without
daylight saving time; other rules can specify simple daylight saving
regimes. See Specifying the Time Zone with TZ.

4.10.11 Authors of parse_datetime

parse_datetime started life as getdate, as originally
implemented by Steven M. Bellovin
(smb@research.att.com) while at the University of North Carolina
at Chapel Hill. The code was later tweaked by a couple of people on
Usenet, then completely overhauled by Rich $alz (rsalz@bbn.com)
and Jim Berets (jberets@bbn.com) in August, 1990. Various
revisions for the GNU system were made by David MacKenzie, Jim Meyering,
Paul Eggert and others, including renaming it to get_date to
avoid a conflict with the alternative Posix function getdate,
and a later rename to parse_datetime. The Posix function
getdate can parse more locale-specific dates using
strptime, but relies on an environment variable and external
file, and lacks the thread-safety of parse_datetime.

5.1 Preparation

To use `Libshishi', you have to perform some changes to your sources
and the build system. The necessary changes are small and explained
in the following sections. At the end of this chapter, it is
described how the library is initialized, and how the requirements of
the library are verified.

A faster way to find out how to adapt your application for use with
`Libshishi' may be to look at the examples at the end of this manual
(see Examples).

5.1.1 Header

All interfaces (data types and functions) of the library are defined
in the header file `shishi.h'. You must include this in all programs
using the library, either directly or through some other header file,
like this:

#include <shishi.h>

The name space of `Libshishi' is shishi_* for function names,
Shishi* for data types and SHISHI_* for other symbols. In
addition the same name prefixes with one prepended underscore are
reserved for internal use and should never be used by an application.

5.1.2 Initialization

`Libshishi' must be initialized before it can be used. The library is
initialized by calling shishi_init (see Initialization Functions). The resources allocated by the initialization process
can be released if the application no longer has a need to call
`Libshishi' functions, this is done by calling shishi_done.

In order to take advantage of the internationalisation features in
`Libshishi', such as translated error messages, the application must
set the current locale using setlocale before initializing
`Libshishi'.

5.1.3 Version Check

It is often desirable to check that the version of `Libshishi' used is
indeed one which fits all requirements. Even with binary
compatibility new features may have been introduced but due to problem
with the dynamic linker an old version is actually used. So you may
want to check that the version is okay right after program startup.

5.1.4 Building the source

If you want to compile a source file including the `shishi.h' header
file, you must make sure that the compiler can find it in the
directory hierarchy. This is accomplished by adding the path to the
directory in which the header file is located to the compilers include
file search path (via the -I option).

However, the path to the include file is determined at the time the
source is configured. To solve this problem, `Libshishi' uses the
external package pkg-config that knows the path to the
include file and other configuration options. The options that need
to be added to the compiler invocation at compile time are output by
the --cflags option to pkg-config shishi. The
following example shows how it can be used at the command line:

gcc -c foo.c `pkg-config shishi --cflags`

Adding the output of ‘pkg-config shishi --cflags’ to the
compilers command line will ensure that the compiler can find the
`Libshishi' header file.

A similar problem occurs when linking the program with the library.
Again, the compiler has to find the library files. For this to work,
the path to the library files has to be added to the library search path
(via the -L option). For this, the option --libs to
pkg-config shishi can be used. For convenience, this option
also outputs all other options that are required to link the program
with the `Libshishi' libararies (in particular, the ‘-lshishi’
option). The example shows how to link foo.o with the `Libshishi'
library to a program foo.

gcc -o foo foo.o `pkg-config shishi --libs`

Of course you can also combine both examples to a single command by
specifying both options to pkg-config:

5.1.5 Autoconf tests

If you work on a project that uses Autoconf (see GNU Autoconf) to help find installed libraries, the
suggestions in the previous section are not the entire story. There
are a few methods to detect and incorporate Shishi into your Autoconf
based package. The preferred approach, is to use Libtool in your
project, and use the normal Autoconf header file and library tests.

5.1.5.1 Autoconf test via ‘pkg-config’

If your audience is a typical GNU/Linux desktop, you can often assume
they have the ‘pkg-config’ tool installed, in which you can use
its Autoconf M4 macro to find and set up your package for use with
Shishi. The following illustrate this scenario.

5.1.5.2 Standalone Autoconf test using Libtool

If your package uses Libtool(see GNU Libtool), you
can use the normal Autoconf tests to find the Shishi library and rely
on the Libtool dependency tracking to include the proper dependency
libraries (e.g., Libidn). The following illustrate this scenario.

5.1.5.3 Standalone Autoconf test

If your package does not use Libtool, as well as detecting the Shishi
library as in the previous case, you must also detect whatever
dependencies Shishi requires to work (e.g., libidn). Since the
dependencies are in a state of flux, we do not provide an example and
we do not recommend this approach, unless you are experienced
developer.

5.2 Initialization Functions

shishi

— Function: Shishi * shishi ( void)

Initializes the Shishi library, and set up, using
shishi_error_set_outputtype(), the library so that future warnings
and informational messages are printed to stderr. If this function
fails, it may print diagnostic errors to stderr.

Return value: Returns Shishi library handle, or NULL on error.

shishi_server

— Function: Shishi * shishi_server ( void)

Initializes the Shishi library, and set up, using
shishi_error_set_outputtype(), the library so that future warnings
and informational messages are printed to the syslog. If this
function fails, it may print diagnostic errors to the syslog.

Return value: Returns Shishi library handle, or NULL on error.

shishi_done

— Function: void shishi_done (Shishi * handle)

handle: shishi handle as allocated by shishi_init().

Deallocates the shishi library handle. The handle must not be used
in any calls to shishi functions after this.

If there is a default tkts, it is written to the default tkts file
(call shishi_tkts_default_file_set() to change the default tkts
file). If you do not wish to write the default tkts file, close the
default tkts with shishi_tkts_done(handle, NULL) before calling
this function.

shishi_init

— Function: int shishi_init (Shishi ** handle)

handle: pointer to handle to be created.

Create a Shishi library handle, using shishi(), and read the system
configuration file, user configuration file and user tickets from
their default locations. The paths to the system configuration
file is decided at compile time, and is $sysconfdir/shishi.conf.
The user configuration file is $HOME/.shishi/config, and the user
ticket file is $HOME/.shishi/ticket.

The handle is allocated regardless of return values, except for
SHISHI_HANDLE_ERROR which indicates a problem allocating the
handle. (The other error conditions comes from reading the files.)

Create a Shishi library handle, using shishi(), and read the system
configuration file, user configuration file, and user tickets from
the specified locations. If any of usercfgfile or systemcfgfile
is NULL, the file is read from its default location, which for the
system configuration file is decided at compile time, and is
$sysconfdir/shishi.conf, and for the user configuration file is
$HOME/.shishi/config. If the ticket file is NULL, a ticket file is
not read at all.

The handle is allocated regardless of return values, except for
SHISHI_HANDLE_ERROR which indicates a problem allocating the
handle. (The other error conditions comes from reading the files.)

Return value: Returns SHISHI_OK iff successful.

shishi_init_server

— Function: int shishi_init_server (Shishi ** handle)

handle: pointer to handle to be created.

Create a Shishi library handle, using shishi_server(), and read the
system configuration file. The paths to the system configuration
file is decided at compile time, and is $sysconfdir/shishi.conf.

The handle is allocated regardless of return values, except for
SHISHI_HANDLE_ERROR which indicates a problem allocating the
handle. (The other error conditions comes from reading the file.)

Return value: Returns SHISHI_OK iff successful.

shishi_init_server_with_paths

Create a Shishi library handle, using shishi_server(), and read the
system configuration file from specified location. The paths to
the system configuration file is decided at compile time, and is
$sysconfdir/shishi.conf. The handle is allocated regardless of
return values, except for SHISHI_HANDLE_ERROR which indicates a
problem allocating the handle. (The other error conditions comes
from reading the file.)

shishi_cfg_default_userdirectory

The default user directory (used for, e.g. Shishi ticket cache) is
normally computed by appending BASE_DIR ("/.shishi") to the content
of the environment variable $HOME, but can be overridden by
specifying the complete path in the environment variable
SHISHI_HOME.

shishi_cfg_clientkdcetype_set

Set the "client-kdc-etypes" configuration option from given string.
The string contains encryption types (integer or names) separated
by comma or whitespace, e.g. "aes256-cts-hmac-sha1-96
des3-cbc-sha1-kd des-cbc-md5".

5.3 Ticket Set Functions

A “ticket set” is, as the name implies, a collection of tickets.
Functions are provided to read tickets from file into a ticket set, to
query number of tickets in the set, to extract a given ticket from the
set, to search the ticket set for tickets matching certain criterium,
to write the ticket set to a file, etc. High level functions for
performing a initial authentication (see AS Functions) or
subsequent authentication (see TGS Functions) and storing the new
ticket in the ticket set are also provided.

Return value: Returns a ticket handle to the ticketno:th ticket in
the ticket set, or NULL if ticket set is invalid or ticketno is
out of bounds. The first ticket is ticketno 0, the second
ticketno 1, and so on.

shishi_tkts_remove

— Function: int shishi_tkts_remove (Shishi_tkts * tkts, int ticketno)

tkts: ticket set handle as allocated by shishi_tkts().

ticketno: ticket number of ticket in the set to remove. The first
ticket is ticket number 0.

Remove a ticket, indexed by ticketno, in ticket set.

Return value:SHISHI_OK if successful or if ticketno larger than
size of ticket set.

shishi_tkts_find

Search the ticketset sequentially (from ticket number 0 through all
tickets in the set) for a ticket that fits the given
characteristics. If a ticket is found, the hint->startpos field is
updated to point to the next ticket in the set, so this function
can be called repeatedly with the same hint argument in order to
find all tickets matching a certain criterium. Note that if
tickets are added to, or removed from, the ticketset during a query
with the same hint argument, the hint->startpos field must be
updated appropriately.

Here is how you would typically use this function:
Shishi_tkts_hint hint;

Shishi_tkt tkt;

memset(&hint, 0, sizeof(hint));

hint.server = "imap/mail.example.org";

tkt = shishi_tkts_find (shishi_tkts_default(handle), &hint);

if (!tkt)

printf("No ticket found...\n");

else

do_something_with_ticket (tkt);

Return value: Returns a ticket if found, or NULL if no further
matching tickets could be found.

shishi_tkts_get_tgt

Get a ticket granting ticket (TGT) suitable for acquiring ticket
matching the hint. I.e., get a TGT for the server realm in the
hint structure (hint->serverrealm), or the default realm if the
serverrealm field is NULL. Can result in AS exchange.

Currently this function do not implement cross realm logic.

This function is used by shishi_tkts_get(), which is probably what
you really want to use unless you have special needs.

Return value: Returns a ticket granting ticket if successful, or
NULL if this function is unable to acquire on.

shishi_tkts_get

Get a ticket matching given characteristics. This function first
looks in the ticket set for a ticket, then tries to find a
suitable TGT, possibly via an AS exchange, using
shishi_tkts_get_tgt(), and then uses that TGT in a TGS exchange to
get the ticket.

Currently this function does not implement cross realm logic.

Return value: Returns a ticket if found, or NULL if this function
is unable to get the ticket.

5.4 AP-REQ and AP-REP Functions

The “AP-REQ” and “AP-REP” are ASN.1 structures used by application
client and servers to prove to each other who they are. The
structures contain auxilliary information, together with an
authenticator (see Authenticator Functions) which is the real
cryptographic proof. The following illustrates the AP-REQ and AP-REP
ASN.1 structures.

shishi_ap

— Function: int shishi_ap (Shishi * handle, Shishi_ap ** ap)

handle: shishi handle as allocated by shishi_init().

ap: pointer to new structure that holds information about AP exchange

Create a new AP exchange with a random subkey of the default
encryption type from configuration. Note that there is no
guarantee that the receiver will understand that key type, you
should probably use shishi_ap_etype() or shishi_ap_nosubkey()
instead. In the future, this function will likely behave as
shishi_ap_nosubkey() and shishi_ap_nosubkey() will be removed.

Create a new AP exchange using shishi_ap(), and set the ticket and
AP-REQ apoptions using shishi_ap_set_tktoptions(). A random
session key is added to the authenticator, using the same keytype
as the ticket.

Create a new AP exchange using shishi_ap(), and set the ticket,
AP-REQ apoptions and the Authenticator checksum data using
shishi_ap_set_tktoptionsdata(). A random session key is added to
the authenticator, using the same keytype as the ticket.

data: input array with data to store in checksum field in Authenticator.

len: length of input array with data to store in checksum field in
Authenticator.

Create a new AP exchange using shishi_ap(), and set the ticket,
AP-REQ apoptions and the raw Authenticator checksum data field
using shishi_ap_set_tktoptionsraw(). A random session key is added
to the authenticator, using the same keytype as the ticket.

Create a new AP exchange using shishi_ap(), and set the ticket,
AP-REQ apoptions and the Authenticator checksum data using
shishi_ap_set_tktoptionsdata(). A random session key is added to
the authenticator, using the same keytype as the ticket.

Create a new AP exchange using shishi_ap(), and set ticket, options
and authenticator checksum data from the DER encoding of the ASN.1
field using shishi_ap_set_tktoptionsasn1usage(). A random session
key is added to the authenticator, using the same keytype as the
ticket.

Return value: Returns SHISHI_OK iff successful.

shishi_ap_tkt

— Function: Shishi_tkt * shishi_ap_tkt (Shishi_ap * ap)

ap: structure that holds information about AP exchange

Get Ticket from AP exchange.

Return value: Returns the ticket from the AP exchange, or NULL if
not yet set or an error occured.

shishi_ap_authenticator_cksumdata_set

authenticatorcksumdata: input array with data to compute checksum
on and store in Authenticator in AP-REQ.

authenticatorcksumdatalen: length of input array with data to
compute checksum on and store in Authenticator in AP-REQ.

Set the Authenticator Checksum Data in the AP exchange. This is
the data that will be checksumed, and the checksum placed in the
checksum field. It is not the actual checksum field. See also
shishi_ap_authenticator_cksumraw_set.

authenticatorcksumraw: input array with authenticator checksum
field value to set in Authenticator in AP-REQ.

authenticatorcksumrawlen: length of input array with
authenticator checksum field value to set in Authenticator in AP-REQ.

Set the Authenticator Checksum Data in the AP exchange. This is
the actual checksum field, not data to compute checksum on and then
store in the checksum field. See also
shishi_ap_authenticator_cksumdata_set.

keyusage: key usage to use during decryption, for normal
AP-REQ's this is normally SHISHI_KEYUSAGE_APREQ_AUTHENTICATOR,
for AP-REQ's part of TGS-REQ's, this is normally
SHISHI_KEYUSAGE_TGSREQ_APREQ_AUTHENTICATOR.

Decrypt ticket in AP-REQ using supplied key and decrypt
Authenticator in AP-REQ using key in decrypted ticket, and on
success set the Ticket and Authenticator fields in the AP exchange.

Set the encrypted authenticator field in the AP-REP. The encrypted
data is usually created by calling shishi_encrypt() on the DER
encoded authenticator. To save time, you may want to use
shishi_apreq_add_authenticator() instead, which calculates the
encrypted data and calls this function in one step.

5.5 SAFE and PRIV Functions

The “KRB-SAFE” is an ASN.1 structure used by application client and
servers to exchange integrity protected data. The integrity
protection is keyed, usually with a key agreed on via the AP exchange
(see AP-REQ and AP-REP Functions). The following illustrates the
KRB-SAFE ASN.1 structure.

Store checksum value in SAFE. A checksum is usually created by
calling shishi_checksum() on some application specific data using
the key from the ticket that is being used. To save time, you may
want to use shishi_safe_build() instead, which calculates the
checksum and calls this function in one step.

The “KRB-PRIV” is an ASN.1 structure used by application client and
servers to exchange confidential data. The confidentiality is keyed,
usually with a key agreed on via the AP exchange (see AP-REQ and AP-REP Functions). The following illustrates the KRB-PRIV ASN.1
structure.

Store encrypted data in PRIV. The encrypted data is usually
created by calling shishi_encrypt() on some application specific
data using the key from the ticket that is being used. To save
time, you may want to use shishi_priv_build() instead, which
encryptes the data and calls this function in one step.

5.6 Ticket Functions

A Ticket is an ASN.1 structured that can be used to authenticate the
holder to services. It contain an encrypted part, which the ticket
holder cannot see, but can be encrypted by the service, and various
information about the user and service, including an encryption key to
use for the connection. See Ticket (ASN.1) Functions, for more
details on the ASN.1 structure of a ticket.

clientlen: pointer to length of client on output, excluding terminating
zero. May be NULL (to only populate client).

Represent client principal name in Ticket KDC-REP as
zero-terminated string. The string is allocate by this function,
and it is the responsibility of the caller to deallocate it. Note
that the output length clientlen does not include the terminating
zero.

shishi_tkt_clientrealm

client: pointer to newly allocated zero terminated string containing
principal name and realm. May be NULL (to only populate clientlen).

clientlen: pointer to length of client on output, excluding terminating
zero. May be NULL (to only populate client).

Convert cname and realm fields from AS-REQ to printable principal
name format. The string is allocate by this function, and it is
the responsibility of the caller to deallocate it. Note that the
output length clientlen does not include the terminating zero.

serverlen: pointer to length of server on output, excluding terminating
zero. May be NULL (to only populate server).

Represent server principal name in Ticket as zero-terminated
string. The string is allocate by this function, and it is the
responsibility of the caller to deallocate it. Note that the
output length serverlen does not include the terminating zero.

shishi_tkt_flags_add

Add ticket flags to Ticket and EncKDCRepPart. This preserves all
existing options.

Return value: Returns SHISHI_OK iff successful.

shishi_tkt_forwardable_p

— Function: int shishi_tkt_forwardable_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket is forwardable.

The FORWARDABLE flag in a ticket is normally only interpreted by
the ticket-granting service. It can be ignored by application
servers. The FORWARDABLE flag has an interpretation similar to
that of the PROXIABLE flag, except ticket-granting tickets may also
be issued with different network addresses. This flag is reset by
default, but users MAY request that it be set by setting the
FORWARDABLE option in the AS request when they request their
initial ticket-granting ticket.

Return value: Returns non-0 iff forwardable flag is set in ticket.

shishi_tkt_forwarded_p

— Function: int shishi_tkt_forwarded_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket is forwarded.

The FORWARDED flag is set by the TGS when a client presents a
ticket with the FORWARDABLE flag set and requests a forwarded
ticket by specifying the FORWARDED KDC option and supplying a set
of addresses for the new ticket. It is also set in all tickets
issued based on tickets with the FORWARDED flag set. Application
servers may choose to process FORWARDED tickets differently than
non-FORWARDED tickets.

Return value: Returns non-0 iff forwarded flag is set in ticket.

shishi_tkt_proxiable_p

— Function: int shishi_tkt_proxiable_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket is proxiable.

The PROXIABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
When set, this flag tells the ticket-granting server that it is OK
to issue a new ticket (but not a ticket-granting ticket) with a
different network address based on this ticket. This flag is set if
requested by the client on initial authentication. By default, the
client will request that it be set when requesting a
ticket-granting ticket, and reset when requesting any other ticket.

Return value: Returns non-0 iff proxiable flag is set in ticket.

shishi_tkt_proxy_p

— Function: int shishi_tkt_proxy_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket is proxy ticket.

The PROXY flag is set in a ticket by the TGS when it issues a proxy
ticket. Application servers MAY check this flag and at their
option they MAY require additional authentication from the agent
presenting the proxy in order to provide an audit trail.

Return value: Returns non-0 iff proxy flag is set in ticket.

shishi_tkt_may_postdate_p

— Function: int shishi_tkt_may_postdate_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket may be used to grant postdated tickets.

The MAY-POSTDATE flag in a ticket is normally only interpreted by
the ticket-granting service. It can be ignored by application
servers. This flag MUST be set in a ticket-granting ticket in
order to issue a postdated ticket based on the presented ticket. It
is reset by default; it MAY be requested by a client by setting the
ALLOW- POSTDATE option in the KRB_AS_REQ message. This flag does
not allow a client to obtain a postdated ticket-granting ticket;
postdated ticket-granting tickets can only by obtained by
requesting the postdating in the KRB_AS_REQ message. The life
(endtime-starttime) of a postdated ticket will be the remaining
life of the ticket-granting ticket at the time of the request,
unless the RENEWABLE option is also set, in which case it can be
the full life (endtime-starttime) of the ticket-granting
ticket. The KDC MAY limit how far in the future a ticket may be
postdated.

Return value: Returns non-0 iff may-postdate flag is set in ticket.

shishi_tkt_postdated_p

— Function: int shishi_tkt_postdated_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket is postdated.

The POSTDATED flag indicates that a ticket has been postdated. The
application server can check the authtime field in the ticket to
see when the original authentication occurred. Some services MAY
choose to reject postdated tickets, or they may only accept them
within a certain period after the original authentication. When the
KDC issues a POSTDATED ticket, it will also be marked as INVALID,
so that the application client MUST present the ticket to the KDC
to be validated before use.

Return value: Returns non-0 iff postdated flag is set in ticket.

shishi_tkt_invalid_p

— Function: int shishi_tkt_invalid_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket is invalid.

The INVALID flag indicates that a ticket is invalid. Application
servers MUST reject tickets which have this flag set. A postdated
ticket will be issued in this form. Invalid tickets MUST be
validated by the KDC before use, by presenting them to the KDC in a
TGS request with the VALIDATE option specified. The KDC will only
validate tickets after their starttime has passed. The validation
is required so that postdated tickets which have been stolen before
their starttime can be rendered permanently invalid (through a
hot-list mechanism).

Return value: Returns non-0 iff invalid flag is set in ticket.

shishi_tkt_renewable_p

— Function: int shishi_tkt_renewable_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket is renewable.

The RENEWABLE flag in a ticket is normally only interpreted by the
ticket-granting service (discussed below in section 3.3). It can
usually be ignored by application servers. However, some
particularly careful application servers MAY disallow renewable
tickets.

Return value: Returns non-0 iff renewable flag is set in ticket.

shishi_tkt_initial_p

— Function: int shishi_tkt_initial_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket was issued using AS exchange.

The INITIAL flag indicates that a ticket was issued using the AS
protocol, rather than issued based on a ticket-granting ticket.
Application servers that want to require the demonstrated knowledge
of a client's secret key (e.g. a password-changing program) can
insist that this flag be set in any tickets they accept, and thus
be assured that the client's key was recently presented to the
application client.

Return value: Returns non-0 iff initial flag is set in ticket.

shishi_tkt_pre_authent_p

— Function: int shishi_tkt_pre_authent_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket was pre-authenticated.

The PRE-AUTHENT and HW-AUTHENT flags provide additional information
about the initial authentication, regardless of whether the current
ticket was issued directly (in which case INITIAL will also be set)
or issued on the basis of a ticket-granting ticket (in which case
the INITIAL flag is clear, but the PRE-AUTHENT and HW-AUTHENT flags
are carried forward from the ticket-granting ticket).

Return value: Returns non-0 iff pre-authent flag is set in ticket.

shishi_tkt_hw_authent_p

— Function: int shishi_tkt_hw_authent_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket is authenticated using a hardware token.

The PRE-AUTHENT and HW-AUTHENT flags provide additional information
about the initial authentication, regardless of whether the current
ticket was issued directly (in which case INITIAL will also be set)
or issued on the basis of a ticket-granting ticket (in which case
the INITIAL flag is clear, but the PRE-AUTHENT and HW-AUTHENT flags
are carried forward from the ticket-granting ticket).

Return value: Returns non-0 iff hw-authent flag is set in ticket.

shishi_tkt_transited_policy_checked_p

The application server is ultimately responsible for accepting or
rejecting authentication and SHOULD check that only suitably
trusted KDCs are relied upon to authenticate a principal. The
transited field in the ticket identifies which realms (and thus
which KDCs) were involved in the authentication process and an
application server would normally check this field. If any of these
are untrusted to authenticate the indicated client principal
(probably determined by a realm-based policy), the authentication
attempt MUST be rejected. The presence of trusted KDCs in this list
does not provide any guarantee; an untrusted KDC may have
fabricated the list.

While the end server ultimately decides whether authentication is
valid, the KDC for the end server's realm MAY apply a realm
specific policy for validating the transited field and accepting
credentials for cross-realm authentication. When the KDC applies
such checks and accepts such cross-realm authentication it will set
the TRANSITED-POLICY-CHECKED flag in the service tickets it issues
based on the cross-realm TGT. A client MAY request that the KDCs
not check the transited field by setting the
DISABLE-TRANSITED-CHECK flag. KDCs are encouraged but not required
to honor this flag.

Application servers MUST either do the transited-realm checks
themselves, or reject cross-realm tickets without TRANSITED-POLICY-
CHECKED set.

shishi_tkt_ok_as_delegate_p

— Function: int shishi_tkt_ok_as_delegate_p (Shishi_tkt * tkt)

tkt: input variable with ticket info.

Determine if ticket is ok as delegated ticket.

The copy of the ticket flags in the encrypted part of the KDC reply
may have the OK-AS-DELEGATE flag set to indicates to the client
that the server specified in the ticket has been determined by
policy of the realm to be a suitable recipient of delegation. A
client can use the presence of this flag to help it make a decision
whether to delegate credentials (either grant a proxy or a
forwarded ticket- granting ticket) to this server. It is
acceptable to ignore the value of this flag. When setting this
flag, an administrator should consider the security and placement
of the server on which the service will run, as well as whether the
service requires the use of delegated credentials.

Send AS-REQ and receive AS-REP or KRB-ERROR. This is the initial
authentication, usually used to acquire a Ticket Granting Ticket.
The hint structure can be used to set, e.g., parameters for TLS
authentication.

Return value: Returns SHISHI_OK iff successful.

shishi_as_sendrecv

— Function: int shishi_as_sendrecv (Shishi_as * as)

as: structure that holds information about AS exchange

Send AS-REQ and receive AS-REP or KRB-ERROR. This is the initial
authentication, usually used to acquire a Ticket Granting Ticket.

5.8 TGS Functions

The Ticket Granting Service (TGS) is used to get subsequent tickets,
authenticated by other tickets (so called ticket granting tickets).
The following illustrates the TGS-REQ and TGS-REP ASN.1 structures.

Send TGS-REQ and receive TGS-REP or KRB-ERROR. This is the
subsequent authentication, usually used to acquire server tickets.
The hint structure can be used to set, e.g., parameters for TLS
authentication.

Return value: Returns SHISHI_OK iff successful.

shishi_tgs_sendrecv

— Function: int shishi_tgs_sendrecv (Shishi_tgs * tgs)

tgs: structure that holds information about TGS exchange

Send TGS-REQ and receive TGS-REP or KRB-ERROR. This is the
subsequent authentication, usually used to acquire server tickets.

serverlen: pointer to length of server on output, excluding terminating
zero. May be NULL (to only populate server).

Represent server principal name in Ticket as zero-terminated
string. The string is allocate by this function, and it is the
responsibility of the caller to deallocate it. Note that the
output length serverlen does not include the terminating zero.

Set the encrypted enc-part field in the Ticket. The encrypted data
is usually created by calling shishi_encrypt() on the DER encoded
enc-part. To save time, you may want to use
shishi_ticket_add_enc_part() instead, which calculates the
encrypted data and calls this function in one step.

clientlen: pointer to length of client on output, excluding terminating
zero. May be NULL (to only populate client).

Represent client principal name in EncTicketPart as zero-terminated
string. The string is allocate by this function, and it is the
responsibility of the caller to deallocate it. Note that the
output length clientlen does not include the terminating zero.

shishi_encticketpart_clientrealm

encticketpart: EncTicketPart variable to get client name and realm from.

client: pointer to newly allocated zero terminated string containing
principal name and realm. May be NULL (to only populate clientlen).

clientlen: pointer to length of client on output, excluding terminating
zero. May be NULL (to only populate client).

Convert cname and realm fields from EncTicketPart to printable
principal name format. The string is allocate by this function,
and it is the responsibility of the caller to deallocate it. Note
that the output length clientlen does not include the terminating
zero.

5.10 AS/TGS Functions

The Authentication Service (AS) is used to get an initial ticket using
e.g. your password. The Ticket Granting Service (TGS) is used to get
subsequent tickets using other tickets. Protocol wise the procedures
are very similar, which is the reason they are described together.
The following illustrates the AS-REQ, TGS-REQ and AS-REP, TGS-REP
ASN.1 structures. Most of the functions use the mnemonic “KDC”
instead of either AS or TGS, which means the function operates on both
AS and TGS types. Only where the distinction between AS and TGS is
important are the AS and TGS names used. Remember, these are
low-level functions, and normal applications will likely be satisfied
with the AS (see AS Functions) and TGS (see TGS Functions)
interfaces, or the even more high-level Ticket Set (see Ticket Set Functions) interface.

Derive the salt that should be used when deriving a key via
shishi_string_to_key() for an AS exchange. Currently this searches
for PA-DATA of type SHISHI_PA_PW_SALT in the AS-REP and returns it
if found, otherwise the salt is derived from the client name and
realm in AS-REQ.

Verify that KDC-REQ.req-body.nonce and EncKDCRepPart.nonce fields
matches. This is one of the steps that has to be performed when
processing a KDC-REQ and KDC-REP exchange.

Return value: Returns SHISHI_OK if successful,
SHISHI_NONCE_LENGTH_MISMATCH if the nonces have different lengths
(usually indicates that buggy server truncated nonce to 4 bytes),
SHISHI_NONCE_MISMATCH if the values differ, or an error code.

authenticator: input variable with Authenticator from AP-REQ in KDC-REQ.

oldenckdcreppart: input variable with EncKDCRepPart used in request.

enckdcreppart: output variable that holds new EncKDCRepPart.

Process a TGS client exchange and output decrypted EncKDCRepPart
which holds details for the new ticket received. This function
simply derives the encryption key from the ticket used to construct
the TGS request and calls shishi_kdc_process(), which see.

Process an AS client exchange and output decrypted EncKDCRepPart
which holds details for the new ticket received. This function
simply derives the encryption key from the password and calls
shishi_kdc_process(), which see.

Process a KDC client exchange and output decrypted EncKDCRepPart
which holds details for the new ticket received. Use
shishi_kdcrep_get_ticket() to extract the ticket. This function
verifies the various conditions that must hold if the response is
to be considered valid, specifically it compares nonces
(shishi_kdc_check_nonce()) and if the exchange was a AS exchange,
it also compares cname and crealm (shishi_as_check_cname() and
shishi_as_check_crealm()).

Usually the shishi_as_process() and shishi_tgs_process() functions
should be used instead, since they simplify the decryption key
computation.

clientlen: pointer to length of client on output, excluding terminating
zero. May be NULL (to only populate client).

Represent client principal name in KDC-REQ as zero-terminated
string. The string is allocate by this function, and it is the
responsibility of the caller to deallocate it. Note that the
output length clientlen does not include the terminating zero.

client: pointer to newly allocated zero terminated string containing
principal name and realm. May be NULL (to only populate clientlen).

clientlen: pointer to length of client on output, excluding terminating
zero. May be NULL (to only populate client).

Convert cname and realm fields from AS-REQ to printable principal
name format. The string is allocate by this function, and it is
the responsibility of the caller to deallocate it. Note that the
output length clientlen does not include the terminating zero.

realmlen: pointer to length of realm on output, excluding terminating
zero. May be NULL (to only populate realmlen).

Get realm field in KDC-REQ as zero-terminated string. The string
is allocate by this function, and it is the responsibility of the
caller to deallocate it. Note that the output length realmlen
does not include the terminating zero.

serverlen: pointer to length of server on output, excluding terminating
zero. May be NULL (to only populate server).

Represent server principal name in KDC-REQ as zero-terminated
string. The string is allocate by this function, and it is the
responsibility of the caller to deallocate it. Note that the
output length serverlen does not include the terminating zero.

tilllen: pointer to length of till on output, excluding
terminating zero. May be NULL (to only populate tilllen).

Get "till" field (i.e. "endtime") in KDC-REQ, as zero-terminated
string. The string is typically 15 characters long. The string is
allocated by this function, and it is the responsibility of the
caller to deallocate it. Note that the output length realmlen
does not include the terminating zero.

The FORWARDABLE option indicates that the ticket to be issued is to
have its forwardable flag set. It may only be set on the initial
request, or in a subsequent request if the ticket-granting ticket
on which it is based is also forwardable.

The FORWARDED option is only specified in a request to the
ticket-granting server and will only be honored if the
ticket-granting ticket in the request has its FORWARDABLE bit
set. This option indicates that this is a request for
forwarding. The address(es) of the host from which the resulting
ticket is to be valid are included in the addresses field of the
request.

The PROXIABLE option indicates that the ticket to be issued is to
have its proxiable flag set. It may only be set on the initial
request, or in a subsequent request if the ticket-granting ticket
on which it is based is also proxiable.

The PROXY option indicates that this is a request for a proxy. This
option will only be honored if the ticket-granting ticket in the
request has its PROXIABLE bit set. The address(es) of the host
from which the resulting ticket is to be valid are included in the
addresses field of the request.

The ALLOW-POSTDATE option indicates that the ticket to be issued is
to have its MAY-POSTDATE flag set. It may only be set on the
initial request, or in a subsequent request if the ticket-granting
ticket on which it is based also has its MAY-POSTDATE flag set.

The POSTDATED option indicates that this is a request for a
postdated ticket. This option will only be honored if the
ticket-granting ticket on which it is based has its MAY-POSTDATE
flag set. The resulting ticket will also have its INVALID flag set,
and that flag may be reset by a subsequent request to the KDC after
the starttime in the ticket has been reached.

The RENEWABLE option indicates that the ticket to be issued is to
have its RENEWABLE flag set. It may only be set on the initial
request, or when the ticket-granting ticket on which the request is
based is also renewable. If this option is requested, then the
rtime field in the request contains the desired absolute expiration
time for the ticket.

By default the KDC will check the transited field of a
ticket-granting-ticket against the policy of the local realm before
it will issue derivative tickets based on the ticket-granting
ticket. If this flag is set in the request, checking of the
transited field is disabled. Tickets issued without the performance
of this check will be noted by the reset (0) value of the
TRANSITED-POLICY-CHECKED flag, indicating to the application server
that the tranisted field must be checked locally. KDCs are
encouraged but not required to honor the DISABLE-TRANSITED-CHECK
option.

The RENEWABLE-OK option indicates that a renewable ticket will be
acceptable if a ticket with the requested life cannot otherwise be
provided. If a ticket with the requested life cannot be provided,
then a renewable ticket may be issued with a renew-till equal to
the requested endtime. The value of the renew-till field may still
be limited by local limits, or limits selected by the individual
principal or server.

This option is used only by the ticket-granting service. The
ENC-TKT-IN-SKEY option indicates that the ticket for the end server
is to be encrypted in the session key from the additional
ticket-granting ticket provided.

This option is used only by the ticket-granting service. The RENEW
option indicates that the present request is for a renewal. The
ticket provided is encrypted in the secret key for the server on
which it is valid. This option will only be honored if the ticket
to be renewed has its RENEWABLE flag set and if the time in its
renew-till field has not passed. The ticket to be renewed is passed
in the padata field as part of the authentication header.

This option is used only by the ticket-granting service. The
VALIDATE option indicates that the request is to validate a
postdated ticket. It will only be honored if the ticket presented
is postdated, presently has its INVALID flag set, and would be
otherwise usable at this time. A ticket cannot be validated before
its starttime. The ticket presented for validation is encrypted in
the key of the server for which it is valid and is passed in the
padata field as part of the authentication header.

Get pre authentication data (PA-DATA) from KDC-REQ. Pre
authentication data is used to pass various information to KDC,
such as in case of a SHISHI_PA_TGS_REQ padatatype the AP-REQ that
authenticates the user to get the ticket.

Extract TGS pre-authentication data from KDC-REQ. The data is an
AP-REQ that authenticates the request. This function call
shishi_kdcreq_get_padata() with a SHISHI_PA_TGS_REQ padatatype and
DER decode the result (if any).

Add new pre authentication data (PA-DATA) to KDC-REQ. This is used
to pass various information to KDC, such as in case of a
SHISHI_PA_TGS_REQ padatatype the AP-REQ that authenticates the user
to get the ticket. (But also see shishi_kdcreq_add_padata_tgs()
which takes an AP-REQ directly.)

Add TGS pre-authentication data to KDC-REQ. The data is an AP-REQ
that authenticates the request. This functions simply DER encodes
the AP-REQ and calls shishi_kdcreq_add_padata() with a
SHISHI_PA_TGS_REQ padatatype.

Set the encrypted enc-part field in the KDC-REP. The encrypted
data is usually created by calling shishi_encrypt() on the DER
encoded enc-part. To save time, you may want to use
shishi_kdcrep_add_enc_part() instead, which calculates the
encrypted data and calls this function in one step.

5.11 Authenticator Functions

An “Authenticator” is an ASN.1 structure that work as a proof that
an entity owns a ticket. It is usually embedded in the AP-REQ
structure (see AP-REQ and AP-REP Functions), and you most likely
want to use an AP-REQ instead of a Authenticator in normal
applications. The following illustrates the Authenticator ASN.1
structure.

clientlen: pointer to length of client on output, excluding terminating
zero. May be NULL (to only populate client).

Represent client principal name in Authenticator as zero-terminated
string. The string is allocate by this function, and it is the
responsibility of the caller to deallocate it. Note that the
output length clientlen does not include the terminating zero.

shishi_authenticator_clientrealm

authenticator: Authenticator variable to get client name and realm from.

client: pointer to newly allocated zero terminated string containing
principal name and realm. May be NULL (to only populate clientlen).

clientlen: pointer to length of client on output, excluding terminating
zero. May be NULL (to only populate client).

Convert cname and realm fields from Authenticator to printable
principal name format. The string is allocate by this function,
and it is the responsibility of the caller to deallocate it. Note
that the output length clientlen does not include the terminating
zero.

Store checksum value in authenticator. A checksum is usually created
by calling shishi_checksum() on some application specific data using
the key from the ticket that is being used. To save time, you may
want to use shishi_authenticator_add_cksum() instead, which calculates
the checksum and calls this function in one step.

Store subkey value in authenticator. A subkey is usually created
by calling shishi_key_random() using the default encryption type of
the key from the ticket that is being used. To save time, you may
want to use shishi_authenticator_add_subkey() instead, which calculates
the subkey and calls this function in one step.

5.12 KRB-ERROR Functions

The “KRB-ERROR” is an ASN.1 structure that can be returned, instead
of, e.g., KDC-REP or AP-REP, to indicate various error conditions.
Unfortunately, the semantics of several of the fields are ill
specified, so the typically procedure is to extract “e-text” and/or
“e-data” and show it to the user. The following illustrates the
KRB-ERROR ASN.1 structure.

5.13 Cryptographic Functions

Underneath the high-level functions described earlier, cryptographic
operations are happening. If you need to access these cryptographic
primitives directly, this section describes the functions available.

Most cryptographic operations need keying material, and cryptographic
keys have been isolated into it's own data structure
Shishi_key. The following illustrates it's contents, but note
that you cannot access it's elements directly but must use the
accessor functions described below.

outkey: pointer to structure that will hold newly created key information

Create a new Key information structure, and derive the key from
principal name and password using shishi_key_from_name(). The salt
is derived from the principal name by concatenating the decoded
realm and principal.

Return value: Returns SHISHI_OK iff successful.

Applications that run uninteractively may need keying material. In
these cases, the keys are stored in a file, a file that is normally
stored on the local host. The file should be protected from
unauthorized access. The file is in ASCII format and contains keys as
outputed by shishi_key_print. All functions that handle these
keys sets are described now.

shishi_keys

— Function: int shishi_keys (Shishi * handle, Shishi_keys ** keys)

handle: shishi handle as allocated by shishi_init().

keys: output pointer to newly allocated keys handle.

Get a new key set handle.

Return value: Returns SHISHI_OK iff successful.

shishi_keys_done

— Function: void shishi_keys_done (Shishi_keys ** keys)

keys: key set handle as allocated by shishi_keys().

Deallocates all resources associated with key set. The key set
handle must not be used in calls to other shishi_keys_*() functions
after this.

Return value: Returns the key for the server
"SERVICE/HOSTNAMEREALM" (where HOSTNAME is the current system's
hostname), read from the default host keys file (see
shishi_hostkeys_default_file()), or NULL if no key could be found
or an error encountered.

The previous functions require that the filename is known. For some
applications, servers, it makes sense to provide a system default.
These key sets used by server applications are known as “hostkeys”.
Here are the functions that operate on hostkeys (they are mostly
wrappers around generic key sets).

Return value: Returns the key for the server
"SERVICE/HOSTNAMEREALM" (where HOSTNAME is the current system's
hostname), read from the default host keys file (see
shishi_hostkeys_default_file()), or NULL if no key could be found
or an error encountered.

Return value: Returns the key for the server "SERVICE/HOSTNAME"
(where HOSTNAME is the current system's hostname), read from the
default host keys file (see shishi_hostkeys_default_file()), or
NULL if no key could be found or an error encountered.

After creating the key structure, it can be used to encrypt and
decrypt data, calculate checksum on data etc. All available functions
are described now.

shishi_cipher_supported_p

— Function: int shishi_cipher_supported_p (int32_t type)

type: encryption type, see Shishi_etype.

Find out if cipher is supported.

Return value: Return 0 iff cipher is unsupported.

shishi_cipher_name

— Function: const char * shishi_cipher_name (int32_t type)

type: encryption type, see Shishi_etype.

Read humanly readable string for cipher.

Return value: Return name of encryption type,
e.g. "des3-cbc-sha1-kd", as defined in the standards.

shishi_cipher_blocksize

— Function: int shishi_cipher_blocksize (int32_t type)

type: encryption type, see Shishi_etype.

Get block size for cipher.

Return value: Return block size for encryption type, as defined in
the standards.

shishi_cipher_confoundersize

— Function: int shishi_cipher_confoundersize (int32_t type)

type: encryption type, see Shishi_etype.

Get length of confounder for cipher.

Return value: Returns the size of the confounder (random data) for
encryption type, as defined in the standards, or (size_t)-1 on
error (e.g., unsupported encryption type).

shishi_cipher_keylen

— Function: size_t shishi_cipher_keylen (int32_t type)

type: encryption type, see Shishi_etype.

Get key length for cipher.

Return value: Return length of key used for the encryption type, as
defined in the standards.

shishi_cipher_randomlen

— Function: size_t shishi_cipher_randomlen (int32_t type)

type: encryption type, see Shishi_etype.

Get length of random data for cipher.

Return value: Return length of random used for the encryption type,
as defined in the standards, or (size_t)-1 on error (e.g.,
unsupported encryption type).

shishi_cipher_defaultcksumtype

— Function: int shishi_cipher_defaultcksumtype (int32_t type)

type: encryption type, see Shishi_etype.

Get the default checksum associated with cipher.

Return value: Return associated checksum mechanism for the
encryption type, as defined in the standards.

shishi_cipher_parse

— Function: int shishi_cipher_parse (const char * cipher)

cipher: name of encryption type, e.g. "des3-cbc-sha1-kd".

Get cipher number by parsing string.

Return value: Return encryption type corresponding to a string.

shishi_checksum_supported_p

— Function: int shishi_checksum_supported_p (int32_t type)

type: checksum type, see Shishi_cksumtype.

Find out whether checksum is supported.

Return value: Return 0 iff checksum is unsupported.

shishi_checksum_name

— Function: const char * shishi_checksum_name (int32_t type)

type: checksum type, see Shishi_cksumtype.

Get name of checksum.

Return value: Return name of checksum type,
e.g. "hmac-sha1-96-aes256", as defined in the standards.

shishi_checksum_cksumlen

— Function: size_t shishi_checksum_cksumlen (int32_t type)

type: checksum type, see Shishi_cksumtype.

Get length of checksum output.

Return value: Return length of checksum used for the checksum type,
as defined in the standards.

Derive key from a string (password) and salt (commonly
concatenation of realm and principal) for specified key type, and
set the type and value in the given key to the computed values.
The parameter value is specific for each keytype, and can be set if
the parameter information is not available.

Encrypts data as per encryption method using specified
initialization vector and key. The key actually used is derived
using the key usage. If key usage is 0, no key derivation is used.
The OUT buffer must be deallocated by the caller. If IVOUT or
IVOUTLEN is NULL, the updated IV is not saved anywhere.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Encrypts data as per encryption method using specified
initialization vector and key. The key actually used is derived
using the key usage. If key usage is 0, no key derivation is used.
The OUT buffer must be deallocated by the caller. The next IV is
lost, see shishi_encrypt_ivupdate_etype if you need it.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Encrypts data as per encryption method using specified
initialization vector and key. The key actually used is derived
using the key usage. If key usage is 0, no key derivation is used.
The OUT buffer must be deallocated by the caller. The default IV
is used, see shishi_encrypt_iv_etype if you need to alter it. The
next IV is lost, see shishi_encrypt_ivupdate_etype if you need it.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Encrypts data using specified initialization vector and key. The
key actually used is derived using the key usage. If key usage is
0, no key derivation is used. The OUT buffer must be deallocated
by the caller. If IVOUT or IVOUTLEN is NULL, the updated IV is not
saved anywhere.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Encrypts data using specified initialization vector and key. The
key actually used is derived using the key usage. If key usage is
0, no key derivation is used. The OUT buffer must be deallocated
by the caller. The next IV is lost, see shishi_encrypt_ivupdate if
you need it.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Encrypts data using specified key. The key actually used is
derived using the key usage. If key usage is 0, no key derivation
is used. The OUT buffer must be deallocated by the caller. The
default IV is used, see shishi_encrypt_iv if you need to alter it.
The next IV is lost, see shishi_encrypt_ivupdate if you need it.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Decrypts data as per encryption method using specified
initialization vector and key. The key actually used is derived
using the key usage. If key usage is 0, no key derivation is used.
The OUT buffer must be deallocated by the caller. If IVOUT or
IVOUTLEN is NULL, the updated IV is not saved anywhere.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Decrypts data as per encryption method using specified
initialization vector and key. The key actually used is derived
using the key usage. If key usage is 0, no key derivation is used.
The OUT buffer must be deallocated by the caller. The next IV is
lost, see shishi_decrypt_ivupdate_etype if you need it.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Decrypts data as per encryption method using specified key. The
key actually used is derived using the key usage. If key usage is
0, no key derivation is used. The OUT buffer must be deallocated
by the caller. The default IV is used, see shishi_decrypt_iv_etype
if you need to alter it. The next IV is lost, see
shishi_decrypt_ivupdate_etype if you need it.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Decrypts data using specified initialization vector and key. The
key actually used is derived using the key usage. If key usage is
0, no key derivation is used. The OUT buffer must be deallocated
by the caller. If IVOUT or IVOUTLEN is NULL, the updated IV is not
saved anywhere.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Decrypts data using specified initialization vector and key. The
key actually used is derived using the key usage. If key usage is
0, no key derivation is used. The OUT buffer must be deallocated
by the caller. The next IV is lost, see
shishi_decrypt_ivupdate_etype if you need it.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Decrypts data specified key. The key actually used is derived
using the key usage. If key usage is 0, no key derivation is used.
The OUT buffer must be deallocated by the caller. The default IV
is used, see shishi_decrypt_iv if you need to alter it. The next
IV is lost, see shishi_decrypt_ivupdate if you need it.

Note that DECRYPT(ENCRYPT(data)) does not necessarily yield data
exactly. Some encryption types add pad to make the data fit into
the block size of the encryption algorithm. Furthermore, the pad
is not guaranteed to look in any special way, although existing
implementations often pad with the zero byte. This means that you
may have to "frame" data, so it is possible to infer the original
length after decryption. Compare ASN.1 DER which contains such
information.

Fold data into a fixed length output array, with the intent to give
each input bit approximately equal weight in determining the value
of each output bit.

The algorithm is from "A Better Key Schedule For DES-like Ciphers"
by Uri Blumenthal and Steven M. Bellovin,
http://www.research.att.com/~smb/papers/ides.pdf, although the
sample vectors provided by the paper are incorrect.

Derive a key from a key and a constant thusly: DK(KEY, PRFCONSTANT) = SHISHI_RANDOM-TO-KEY(SHISHI_DR(KEY, PRFCONSTANT)).

Return value: Returns SHISHI_OK iff successful.

An easier way to use encryption and decryption if your application
repeatedly calls, e.g., shishi_encrypt_ivupdate, is to use the
following functions. They store the key, initialization vector, etc,
in a context, and the encryption and decryption operations update the
IV within the context automatically.

Initialize a crypto context. This store a key, keyusage,
encryption type and initialization vector in a "context", and the
caller can then use this context to perform encryption via
shishi_crypto_encrypt() and decryption via shishi_crypto_encrypt()
without supplying all those details again. The functions also
takes care of propagating the IV between calls.

When the application no longer need to use the context, it should
deallocate resources associated with it by calling
shishi_crypto_close().

Decrypt data, using information (e.g., key and initialization
vector) from context. The IV is updated inside the context after
this call.

When the application no longer need to use the context, it should
deallocate resources associated with it by calling
shishi_crypto_close().

Return value: Returns SHISHI_OK iff successful.

shishi_crypto_close

— Function: void shishi_crypto_close (Shishi_crypto * ctx)

ctx: crypto context as returned by shishi_crypto().

Deallocate resources associated with the crypto context.

Also included in Shishi is an interface to the really low-level
cryptographic primitives. They map directly on the underlying
cryptographic library used (i.e., Gnulib or Libgcrypt) and is used
internally by Shishi.

Encrypt or decrypt data (depending on decryptp) using ARCFOUR.
The out buffer must be deallocated by the caller.

The "initialization vector" used here is the concatenation of the
sbox and i and j, and is thus always of size 256 + 1 + 1. This is
a slight abuse of terminology, and assumes you know what you are
doing. Don't use it if you can avoid to.

dkLen: intended length in octets of the derived key, a positive integer,
at most (2^32 - 1) * hLen. The DK array must have room for this many
characters.

DK: output derived key, a dkLen-octet string

Derive key using the PBKDF2 defined in PKCS5. PBKDF2 applies a
pseudorandom function to derive keys. The length of the derived key
is essentially unbounded. (However, the maximum effective search
space for the derived key may be limited by the structure of the
underlying pseudorandom function, which is this function is always
SHA1.)

shishi_x509ca_default_file_set

x509cafile: string with new default x509 client certificate file name,
or NULL to reset to default.

Set the default X.509 CA certificate filename used in the library.
The certificate is used during TLS connections with the KDC to
authenticate the KDC. The string is copied into the library, so
you can dispose of the variable immediately after calling this
function.

shishi_x509ca_default_file

— Function: const char * shishi_x509ca_default_file (Shishi * handle)

handle: Shishi library handle create by shishi_init().

Get filename for default X.509 CA certificate.

Return value: Returns the default X.509 CA certificate filename
used in the library. The certificate is used during TLS
connections with the KDC to authenticate the KDC. The string is
not a copy, so don't modify or deallocate it.

shishi_x509cert_default_file_set

x509certfile: string with new default x509 client certificate file name,
or NULL to reset to default.

Set the default X.509 client certificate filename used in the
library. The certificate is used during TLS connections with the
KDC to authenticate the client. The string is copied into the
library, so you can dispose of the variable immediately after
calling this function.

shishi_x509cert_default_file

Return value: Returns the default X.509 client certificate filename
used in the library. The certificate is used during TLS
connections with the KDC to authenticate the client. The string is
not a copy, so don't modify or deallocate it.

shishi_x509key_default_file_set

x509keyfile: string with new default x509 client key file name, or
NULL to reset to default.

Set the default X.509 client key filename used in the library. The
key is used during TLS connections with the KDC to authenticate the
client. The string is copied into the library, so you can dispose
of the variable immediately after calling this function.

shishi_x509key_default_file

Return value: Returns the default X.509 client key filename
used in the library. The key is used during TLS
connections with the KDC to authenticate the client. The string is
not a copy, so don't modify or deallocate it.

5.15 Utility Functions

shishi_realm_default_guess

— Function: char * shishi_realm_default_guess ( void)

Guesses a realm based on getdomainname() (which really is NIS/YP
domain, but if it is set it might be a good guess), or if it fails,
based on gethostname(), or if it fails, the string
"could-not-guess-default-realm". Note that the hostname is not
trimmed off of the data returned by gethostname() to get the domain
name and use that as the realm.

Return value: Returns guessed realm for host as a string that has
to be deallocated with free() by the caller.

shishi_realm_default

— Function: const char * shishi_realm_default (Shishi * handle)

handle: Shishi library handle create by shishi_init().

Get name of default realm.

Return value: Returns the default realm used in the library. (Not
a copy of it, so don't modify or deallocate it.)

shishi_realm_for_server_dns

Find realm for a host using DNS lookups, according to
draft-ietf-krb-wg-krb-dns-locate-03.txt. Since DNS lookups may be
spoofed, relying on the realm information may result in a
redirection attack. In a single-realm scenario, this only achieves
a denial of service, but with cross-realm trust it may redirect you
to a compromised realm. For this reason, Shishi prints a warning,
suggesting that the user should add the proper 'server-realm'
configuration tokens instead.

To illustrate the DNS information used, here is an extract from a
zone file for the domain ASDF.COM:

outlen: pointer to length of out on output, excluding terminating
null. May be NULL (to only populate out).

Represent principal name in ASN.1 structure as null-terminated
string. The string is allocated by this function, and it is the
responsibility of the caller to deallocate it. Note that the
output length outlen does not include the terminating null.

outlen: pointer to length of out on output, excluding terminating
null. May be NULL (to only populate out).

Represent principal name and realm in ASN.1 structure as
null-terminated string. The string is allocated by this function.
It is the responsibility of the caller to deallocate it. Note
that the output length outlen does not include the terminating
null character.

Construct a service principal (e.g., "imap/yxa.extuno.com") based
on supplied service name (i.e., "imap") and the system's hostname as
returned by hostname() (i.e., "yxa.extundo.com"). The string must
be deallocated by the caller.

Simplistic authorization of authzname against encrypted client
principal name inside ticket. For "basic" authentication type,
the principal name must coincide with authzname. The "k5login"
authentication type attempts the MIT/Heimdal method of parsing
the file "~/.k5login" for additional equivalence names.

Return value: Returns 1 if authzname is authorized for services
by the encrypted principal, and 0 otherwise.

Format and print a prompt, and read a password from user. The
password is possibly converted (e.g., converted from Latin-1 to
UTF-8, or processed using Stringprep profile) following any
"stringprocess" keywords in configuration files.

Extract data stored in a ASN.1 field into a newly allocated buffer.
If the field does not exist (i.e., SHISHI_ASN1_NO_ELEMENT), this
function set datalen to 0 and succeeds. Can be useful to read
ASN.1 fields which are marked OPTIONAL in the grammar, if you want
to avoid special error handling in your code.

Return value: Returns SHISHI_OK if successful,
SHISHI_ASN1_NO_VALUE if the field has no value, ot
SHISHI_ASN1_ERROR otherwise.

shishi_asn1_done

— Function: void shishi_asn1_done (Shishi * handle, Shishi_asn1 node)

handle: shishi handle as allocated by shishi_init().

node: ASN.1 node to dellocate.

Deallocate resources associated with ASN.1 structure. Note that
the node must not be used after this call.

5.17 Error Handling

Most functions in `Libshishi' are returning an error if they fail.
For this reason, the application should always catch the error
condition and take appropriate measures, for example by releasing the
resources and passing the error up to the caller, or by displaying a
descriptive message to the user and cancelling the operation.

Some error values do not indicate a system error or an error in the
operation, but the result of an operation that failed properly.

5.17.1 Error Values

Errors are returned as an int. Except for the SHISHI_OK case,
an application should always use the constants instead of their
numeric value. Applications are encouraged to use the constants even
for SHISHI_OK as it improves readability. Possible values are:

SHISHI_OK

This value indicates success. The value of this error is guaranteed
to always be 0 so you may use it in boolean constructs.

5.17.2 Error Functions

shishi_strerror

Return value: Returns a pointer to a statically allocated string
containing a description of the error with the error value err.
This string can be used to output a diagnostic message to the user.

shishi_error

— Function: const char * shishi_error (Shishi * handle)

handle: shishi handle as allocated by shishi_init().

Extract detailed error information string. Note that the memory is
managed by the Shishi library, so you must not deallocate the
string.

Return value: Returns pointer to error information string, that must
not be deallocate by caller.

shishi_error_clear

— Function: void shishi_error_clear (Shishi * handle)

handle: shishi handle as allocated by shishi_init().

Clear the detailed error information string. See shishi_error()
for how to access the error string, and shishi_error_set() and
shishi_error_printf() for how to set the error string. This
function is mostly for Shishi internal use, but if you develop an
extension of Shishi, it may be useful to use the same error
handling infrastructure.

Set the detailed error information string to specified string. The
string is copied into the Shishi internal structure, so you can
deallocate the string passed to this function after the call. This
function is mostly for Shishi internal use, but if you develop an
extension of Shishi, it may be useful to use the same error
handling infrastructure.

shishi_error_printf

Set the detailed error information string to a printf formatted
string. This function is mostly for Shishi internal use, but if
you develop an extension of Shishi, it may be useful to use the
same error handling infrastructure.

5.18 Examples

This section will be extended to contain walk-throughs of example code
that demonstrate how `Shishi' is used to write your own applications
that support Kerberos 5. The rest of the current section consists of
some crude hints for the example client/server applications that is
part of Shishi, taken from an email but saved here for lack of a
better place to put it.

There are two programs: 'client' and 'server' in src/.

The client output an AP-REQ, waits for an AP-REP, and then simply
reads data from stdin.

The server waits for an AP-REQ, parses it and prints an AP-REP, and
then read data from stdin.

Both programs accept a Kerberos server name as the first command line
argument. Your KDC must know this server, since the client tries to
get a ticket for it (first it gets a ticket granting ticket for the
default username), and you must write the key for the server into
/usr/local/etc/shishi.keys on the Shishi format, e.g.:

5.19 Kerberos Database Functions

Shisa is a separate and standalone library from Shishi
(see Introduction to Shisa). If you only wish to manipulate the
information stored in the Kerberos user database used by Shishi, you
do not need to link or use the Shishi library at all. However, you
may find it useful to combine the two libraries.

For two real world examples on using the Shisa library, refer to
src/shisa.c (Shisa command line tool) and src/kdc.c
(part of Shishid server).

Shisa uses two ‘struct’s to carry information. The first,
Shisa_principal, is used to hold information about principals.
The struct does not contain pointers to strings etc, so the library
assumes the caller is responsible for allocating and deallocating the
struct itself. Each such struct is (uniquely) identified by the
combination of principal name and realm name.

struct Shisa_principal
{
int isdisabled;
uint32_t kvno;
time_t notusedbefore;
time_t lastinitialtgt; /* time of last initial request for a TGT */
time_t lastinitialrequest; /* time of last initial request */
time_t lasttgt; /* time of issue for the newest TGT used */
time_t lastrenewal; /* time of the last renewal */
time_t passwordexpire; /* time when the password will expire */
time_t accountexpire; /* time when the account will expire. */
};
typedef struct Shisa_principal Shisa_principal;

The second structure is called Shisa_key and hold information
about cryptographic keys. Because the struct contain pointers, and
the caller cannot know how many keys a principal have, the Shisa
library manages memory for the struct. The library allocate the
structs, and the pointers within them. The caller may deallocate
them, but it is recommended to use shisa_key_free or
shisa_keys_free instead. Note that each principal may have
multiple keys.

Shisa is typically initialized by calling shisa_init, and
deinitialized (when the application no longer need to use Shisa,
typically when it shuts down) by calling shisa_done, but here
are the complete (de)initialization interface functions.

shisa

— Function: Shisa * shisa ( void)

Initializes the Shisa library. If this function fails, it may
print diagnostic errors to stderr.

Return value: Returns Shisa library handle, or NULL on error.

shisa_done

— Function: void shisa_done (Shisa * dbh)

Deallocates the shisa library handle. The handle must not be used
in any calls to shisa functions after this.

shisa_init

— Function: int shisa_init (Shisa ** dbh)

dbh: pointer to library handle to be created.

Create a Shisa library handle, using shisa(), and read the system
configuration file from their default locations. The paths to the
default system configuration file is decided at compile time
($sysconfdir/shisa.conf).

The handle is allocated regardless of return values, except for
SHISA_INIT_ERROR which indicates a problem allocating the handle.
(The other error conditions comes from reading the files.)

Return value: Returns SHISA_OK iff successful.

shisa_init_with_paths

Create a Shisa library handle, using shisa(), and read the system
configuration file indicated location (or the default location, if
NULL). The paths to the default system configuration file is
decided at compile time ($sysconfdir/shisa.conf).

The handle is allocated regardless of return values, except for
SHISA_INIT_ERROR which indicates a problem allocating the handle.
(The other error conditions comes from reading the files.)

Return value: Returns SHISA_OK iff successful.

The default configuration file is typically read automatically by
calling shisa_init, but if you wish to manually access the
Shisa configuration file functions, here is the complete interface.

shisa_cfg_db

— Function: int shisa_cfg_db (Shisa * dbh, const char * value)

dbh: Shisa library handle created by shisa().

value: string with database definition.

Setup and open a new database. The syntax of the value parameter
is "TYPE[ LOCATION[ PARAMETER]]", where TYPE is one of the
supported database types (e.g., "file") and LOCATION and PARAMETER
are optional strings passed to the database during initialization.
Neither TYPE nor LOCATION can contain " " (SPC), but PARAMETER may.

Return Value: Returns SHISA_OK if database was parsed and open
successfully.

shisa_cfg

— Function: int shisa_cfg (Shisa * dbh, const char * option)

dbh: Shisa library handle created by shisa().

option: string with shisa library option.

Configure shisa library with given option.

Return Value: Returns SHISA_OK if option was valid.

shisa_cfg_from_file

— Function: int shisa_cfg_from_file (Shisa * dbh, const char * cfg)

dbh: Shisa library handle created by shisa().

cfg: filename to read configuration from.

Configure shisa library using configuration file.

Return Value: Returns SHISA_OK iff successful.

shisa_cfg_default_systemfile

— Function: const char * shisa_cfg_default_systemfile (Shisa * dbh)

dbh: Shisa library handle created by shisa().

Return value: Return system configuration filename.

The core part of the Shisa interface follows. The typical procedure
is to use shisa_principal_find to verify that a specific
principal exists, and to extract some information about it, and then
use shisa_keys_find to get the cryptographic keys for the
principal, usually suppliying some hints as to which of all keys you
are interested in (e.g., key version number and encryption algorithm
number).

ph: Pointer to principal structure with information to store in database.

Modify information stored for given PRINCIPALREALM. Note that it
is usually a good idea to only set the fields in ph that you
actually want to update. Specifically, first calling
shisa_principal_find() to get the current information, then
modifying one field, and calling shisa_principal_update() is not
recommended in general, as this will 1) overwrite any modifications
made to other fields between the two calls (by other processes) and
2) will cause all values to be written again, which may generate
more overhead.

Return value: Returns SHISA_OK if successful, SHISA_NO_REALM if
the indicated realm does not exist, SHISA_NO_PRINCIPAL if the
indicated principal does not exist, or an error code.

hint: Pointer to Shisa key structure with hints on matching the key
to modify, may be NULL to match all keys.

keys: pointer to newly allocated array with Shisa key structures.

nkeys: pointer to number of newly allocated Shisa key structures in keys.

Iterate through keys for given PRINCIPALREALM and extract any keys
that match hint. Not all elements of hint need to be filled out,
only use the fields you are interested in. For example, if you
want to extract all keys with an etype of 3 (DES-CBC-MD5), set the
key->etype field to 3, and set all other fields to 0.

oldkey: Pointer to Shisa key structure with hints on matching the key
to modify.

newkey: Pointer to Shisa key structure with new values for the
key, note that all fields are used (and not just the ones specified
by oldkey).

Modify data about a key in the database, for the given
PRINCIPALREALM. First the oldkey is used to locate the key to
update (similar to shisa_keys_find()), then that key is modified to
contain whatever information is stored in newkey. Not all
elements of oldkey need to be filled out, only enough as to
identify the key uniquely. For example, if you want to modify the
information stored for the only key with an etype of 3
(DES-CBC-MD5), set the key->etype field to 3, and set all other
fields to 0.

Return value: Returns SHISA_OK on success, SHISA_NO_KEY if no key
could be identified, and SHISA_MULTIPLE_KEY_MATCH if more than one
key matched the given criteria, or an error code.

key: Pointer to Shisa key structure with hints on matching the key
to remove.

Remove a key, matching the hints in key, from the Shisa database
for the user PRINCIPALREALM. Not all elements of key need to be
filled out, only those you are interested in. For example, if you
want to remove the only key with an etype of 3 (DES-CBC-MD5), set
the key->etype field to 3, and set all other fields to 0.

Return value: Returns SHISA_OK on success, SHISA_NO_KEY if no key
could be identified, and SHISA_MULTIPLE_KEY_MATCH if more than one
key matched the given criteria, or an error code.

shisa_key_free

— Function: void shisa_key_free (Shisa * dbh, Shisa_key * key)

dbh: Shisa library handle created by shisa().

key: Pointer to Shisa key structure to deallocate.

Deallocate the fields of a Shisa key structure, and the structure
itself.

Deallocate each element of an array with Shisa database keys, using
shisa_key_free().

Error handling is similar to that for Shishi in general (see Error Handling), i.e., you invoke shisa_strerror on the integer
return value received by some function, if the return value is
non-zero. Below is the complete interface.

shisa_strerror

— Function: const char * shisa_strerror (int err)

err: shisa error code

Return value: Returns a pointer to a statically allocated string
containing a description of the error with the error value err.
This string can be used to output a diagnostic message to the user.

5.20 Generic Security Service

As an alternative to the native Shishi programming API, it is possible
to program Shishi through the Generic Security Services (GSS) API.
The advantage of using GSS-API in your security application, instead
of the native Shishi API, is that it will be easier to port your
application between different Kerberos 5 implementations, and even
beyond Kerberos 5 to different security systems, that support GSS-API.
In the free software world, however, almost the only widely used
security system that supports GSS-API is Kerberos 5, so the last
advantage is somewhat academic. But if you are porting applications
using GSS-API for other Kerberos 5 implementations, or want a more
mature and stable API than the native Shishi API, you may find using
Shishi's GSS-API interface compelling. Note that GSS-API only offer
basic services, for more advanced uses you must use the native API.

Since the GSS-API is not specific to Shishi, it is distributed
independently from Shishi. Further information on the GSS project can
be found at http://www.gnu.org/software/gss/.

Appendix A Criticism of Kerberos

The intention with this section is to discuss various problems with
Kerberos 5, so you can form a conscious decision how to deploy and use
Shishi correctly in your organization. Currently the issues below are
condensed, and mostly serve as a reminder for the author to elaborate
on them.

Non-formal specification. Unclear on the etype to use for session
keys (etype in request or database?). Unclear on how to populate some
“evident” fields (e.g., cname in tickets for AS-REQ, or crealm,
cname, realm, sname, ctime and cusec in KRB-ERROR). Unclear error
code semantics (e.g., logic for when to use S_PRINCIPAL_UNKNOWN
absent). Some KRB-ERROR fields are required, but can't be usefully
populated in some situations, and no guidance is given on what they
should contain.

RFC 1510/1510bis incompatibilities. NULL enctype removed without
discussion, and it is still used by some 1964 GSSAPI implementations.
KRB_SAFE text (3.4.1) says the checksum is generated using the session
or sub-session key, which contradicts itself (compare section 3.2.6)
and also RFC 1510, which both allow the application to define the key.
Verification of KRB_SAFE now require the key to be compatible with the
(sub-)session key, in 1510 the only requirement was that it was
collision proof.

Problems with crypto specification. It uses the word “random” many
times, but there is no discussion on the randomness requirements.
Practical experience indicate it is impossible to use true randomness
for all “random” fields, and no implementation does this. A post by
Don Davis on the ietf-krb-wg list tried to provide insight, but the
information was never added to the specification.

B.1 STARTTLS protected KDC exchanges

Shishi is able to “upgrade” TCP communications with the KDC to use
the Transport Layer Security (TLS) protocol. The TLS protocol offers
integrity and privacy protected exchanges. TLS also offers
authentication using username and passwords, X.509 certificates, or
OpenPGP certificates. Kerberos 5 claims to offer some of these
features, although it is not as rich as the TLS protocol. An
inconclusive list of the motivation for using TLS is given below.

Server authentication of the KDC to the client.
In traditional Kerberos 5, KDC authentication is only proved as a side
effect that the KDC knows your encryption key (i.e., your password).

Client authentication against KDC.
Kerberos 5 assume the user knows a key (usually in the form of a
password). Sometimes external factors make this hard to fulfill. In
some situations, users are equipped with smart cards with a RSA
authentication key. In others, users have a OpenPGP client on their
desktop, with a public OpenPGP key known to the server. In some
situations, the policy may be that password authentication may only be
done through SRP.

Kerberos exchanges are privacy protected.
Part of many Kerberos packets are transfered without privacy
protection (i.e., encryption). That part contains information, such
as the client principal name, the server principal name, the
encryption types supported by the client, the lifetime of tickets,
etc. Revealing such information is, in some threat models, considered
a problem. Thus, this enables “anonymity”.

Prevents downgrade attacks affecting encryption types.
The encryption type of the ticket in KDC-REQ are sent in the clear in
Kerberos 5. This allows an attacker to replace the encryption type
with a compromised mechanisms, e.g. 56-bit DES. Since clients in
general cannot know the encryption types other servers support, it is
difficult for the client to detect if there was a man-in-the-middle or
if the remote server simply did not support a stronger mechanism.
Clients may chose to refuse 56-bit DES altogether, but in some
environments this leads to operational difficulties.

TLS is well-proved and the protocol is studied by many parties.
This is an advantage in network design, where TLS is often already
assumed as part of the solution since it is used to protect HTTP,
IMAP, SMTP etc. In some threat models, the designer prefer to reduce
the number of protocols that can hurt the overall system security if
they are compromised.

Other reasons for using TLS exists.

B.1.1 TCP/IP transport with TLS upgrade (STARTTLS)

RFC 1510bis requires Kerberos servers (KDCs) to accept TCP requests.
Each request and response is prefixed by a 4 octet integer in network
byte order, indicating the length of the packet. The high bit of the
length was reserved for future expansion, and servers that do not
understand how to interpret a set high bit must return a
KRB-ERROR with a KRB_ERR_FIELD_TOOLONG and close the TCP
stream.

The TCP/IP transport with TLS upgrade (STARTTLS) uses this reserved
bit as follows. First we define a new extensible typed hole for
Kerberos 5 messages, because we used the only reserved bit. It is
thus prudent to offer future extensions on our proposal. Secondly we
reserve two values in this new typed hole, and described how they are
used to implement STARTTLS.

B.1.2 Extensible typed hole based on reserved high bit

When the high bit is set, the remaining 31 bits of the 4 octets are
treated as an extensible typed hole, and thus form a 31 bit integer
enumerating various extensions. Each of the values indicate a
specific extended operation mode, two of which are used and defined
here, and the rest are left for others to use. If the KDC do not
understand a requested extension, it MUST return a KRB-ERROR
with a KRB_ERR_FIELD_TOOLONG value (prefixed by the 4 octet
length integer, with the high bit clear, as usual) and close the TCP
stream.

Meaning of the 31 lower bits in the 4 octet field, when the high bit
is set:

B.1.3 STARTTLS requested by client (extension mode 1)

When this is sent by the client, the client is requesting the server
to start TLS negotiation on the TCP stream. The client MUST NOT start
TLS negotiation immediately. Instead, the client wait for either a
KRB-ERROR (sent normally, prefixed by a 4 octet length integer)
indicating the server do not understand the set high bit, or 4 octet
which is to interpreted as an integer in network byte order, where the
high bit is set and the remaining 31 bit are interpreted as an integer
specifying the “STARTTLS request accepted by server”. In the first
case, the client infer that the server do not understand (or wish to
support) STARTTLS, and can re-try using normal TCP, if unprotected
Kerberos 5 exchanges are acceptable to the client policy. In the
latter case, it should invoke TLS negotiation on the stream. If any
other data is received, the client MUST close the TCP stream.

B.1.4 STARTTLS request accepted by server (extension mode 2)

This 4 octet message should be sent by the server when it has received
the previous 4 octet message. The message is an acknowledgment of the
client's request to initiate STARTTLS on the channel. The server MUST
then invoke a TLS negotiation.

B.1.5 Proceeding after successful TLS negotiation

If the TLS negotiation ended successfully, possibly also considering
client or server policies, the exchange within the TLS protected
stream is performed like normal UDP Kerberos 5 exchanges, i.e., there
is no TCP 4 octet length field before each packet. Instead each
Kerberos packet MUST be sent within one TLS record, so the application
can use the TLS record length as the Kerberos 5 packet length.

B.1.6 Proceeding after failed TLS negotiation

If the TLS negotiation fails, possibly due to client or server policy
(e.g., inadequate support of encryption types in TLS, or lack of
client or server authentication) the entity that detect the failure
MUST disconnected the connection. It is expected that any error
messages that explain the error condition is transfered by TLS.

B.1.7 Interaction with KDC addresses in DNS

Administrators for a KDC may announce the KDC address by placing SRV
records in DNS for the realm, as described in
draft-ietf-krb-wg-krb-dns-locate-03.txt. That document mention
TLS, but do not reference any work that describe how KDCs uses TLS.
Until further clarified, consider the TLS field in that document to
refer to implementation supporting this STARTTLS protocol.

B.1.8 Using TLS authentication logic in Kerberos

The server MAY consider the authentication performed by the TLS
exchange as sufficient to issue Kerberos 5 tickets to the client,
without requiring, e.g., pre-authentication. However, it is not an
error to require or use pre-authentication as well.

The client may also indicate that it wishes to use TLS both for
authentication and data protection by using the ‘NULL’ encryption
type in its request. The server can decide from its local policy
whether or not issuing tickets based solely on TLS authentication, and
whether ‘NULL’ encryption within TLS, is acceptable or not. This
mode is currently under investigation.

B.1.9 Security considerations

Because the initial token is not protected, it is possible for an
active attacker to make it appear to the client that the server do not
support this extension. It is up to client configuration to disallow
non-TLS connections, if this vulnerability is deemed unacceptable.
For interoperability, we suggest the default behaviour should be to
allow automatic fallback to TCP or UDP.

The security considerations of both TLS and Kerberos 5 are inherited.
Using TLS for authentication and/or data protection together with
Kerberos alter the authentication logic fundamentally. Thus, it may
be that even if the TLS and Kerberos 5 protocols and implementations
were secure, the combination of TLS and Kerberos 5 described here
could be insecure.

No channel bindings are provided in the Kerberos messages. It is an
open question whether, and how, this should be fixed.

B.2.1 Command Names and Codes

B.2.2 Command Meanings

IAC SB ENCRYPT IS AES_CCM AES_CCM_INFO <M> <L> <nonce> IAC SE

The sender of this command select desired M and L parameters, and
nonce, as described in RFC 3610, and sends it to the other side of the
connection. The parameters and the nonce are sent in clear text.
Only the side of the connection that is WILL ENCRYPT may send the
AES_CCM_INFO command.

IAC SB ENCRYPT REPLY AES_CCM AES_CCM_INFO_BAD IAC SE

The sender of this command reject the parameters received in the
AES_CCM_INFO command. Only the side of the connection that is DO
ENCRYPT may send the AES_CCM_INFO_BAD command. The command MUST be
sent if the nonce field length does not match the selected value for
L. The command MAY be sent if the receiver do not accept the
parameters for reason such as policy. No capability is provided to
negotiate these parameters.

IAC SB ENCRYPT REPLY AES_CCM AES_CCM_INFO_OK IAC SE

The sender of this command accepts the parameters received in the
AES_CCM_INFO command. Only the side of the connection that is DO
ENCRYPT may send the AES_CCM_INFO_BAD command. The command MUST NOT
be sent if the nonce field length does not match the selected value
for L.

B.2.3 Implementation Rules

Once a AES_CCM_INFO_OK command has been received, the WILL ENCRYPT
side of the connection should do keyid negotiation using the ENC_KEYID
command. Once the keyid negotiation has successfully identified a
common keyid, then START and END commands may be sent by the side of
the connection that is WILL ENCRYPT. Data will be encrypted using the
AES-CCM algorithm, with the negotiated nonce and parameters M and L.
After each successful encryption and decryption, the nonce is treated
as an integer in network byte order, and incremented by one.

If encryption (decryption) is turned off and back on again, and the
same keyid is used when re-starting the encryption (decryption), the
intervening clear text must not change the state of the encryption
(decryption) machine. In particular, the AES-CCM nonce must not be
re-set.

If a START command is sent (received) with a different keyid, the
encryption (decryption) machine must be re-initialized immediately
following the end of the START command with the new key and the
parameters sent (received) in the last AES_CCM_INFO command.

If a new AES_CCM_INFO command is sent (received), and encryption
(decryption) is enabled, the encryption (decryption) machine must be
re-initialized immediately following the end of the AES_CCM_INFO
command with the new nonce and parameters, and the keyid sent
(received) in the last START command.

If encryption (decryption) is not enabled when a AES_CCM_INFO command
is sent (received), the encryption (decryption) machine must be re-
initialized after the next START command, with the keyid sent
(received) in that START command, and the nonce and parameters sent
(received) in this AES_CCM_INFO command.

At all times MUST each end make sure that a AES-CCM nonce is not used
twice under the same encryption key. The rules above help accomplish
this in an interoperable way.

B.2.4 Integration with the AUTHENTICATION telnet option

<<This section is slightly complicated. Can't we simplify this?>>

As noted in the telnet ENCRYPTION option specifications, a keyid value
of zero indicates the default encryption key, as might be derived from
the telnet AUTHENTICATION option. If the default encryption key
negotiated as a result of the telnet AUTHENTICATION option contains
less than 32 bytes (corresponding to two 128 bit keys), then the
AES_CCM option MUST NOT be offered or used as a valid telnet
encryption option. Furthermore, depending on policy for key lengths,
the AES_CCM option MAY be disabled if the default encryption key
contain less than 48 bytes (for two 192 bit keys), or less than 64
bytes (for two 256 bit keys), as well.

The available encrypt key data is divided on two halves, where the
first half is used to encrypt data sent from the server (decrypt data
received by the client), and the second half is used to encrypt data
sent from the client (decrypt data received by the server).

Note that the above algorithm assumes that the AUTHENTICATION
mechanism generate keying material suitable for AES-CCM as used in
this specification. This is not necessarily true in general, but we
specify this behaviour as the default since it is true for most
authentication systems in popular use today. New telnet
AUTHENTICATION mechanisms may specify alternative methods for
determining the keys to be used for this cipher suite in their
specification, if the session key negotiated by that authentication
mechanism is not a DES key and where this algorithm may not be
safely used.

Kerberos 5 authentication clarification: The key used to encrypt data
from the client to the server is taken from the sub-session key in the
AP-REQ. The key used to decrypt data from the server to the client is
taken from the sub-session key in the AP-REP. If mutual
authentication is not negotiated, the key used to encrypt data from
the client to the server is taken from the session key in the ticket,
and the key used to decrypt data from the server to the client is
taken from the sub-session key in the AP-REQ. Leaving the AP-REQ
sub-key field empty MUST disable the AES_CCM option.

B.2.5 Security Considerations

The protocol must be properly and securely implemented. For example,
an implementation should not be vulnerable to various
implementation-specific attacks such as buffer overflows or
side-channel analysis.

We wish to repeat the suggestion from RFC 2946, to investigate in a
STARTTLS approach for Telnet encryption (and also authentication),
when the security level provided by this specification is not
adequate.

B.2.5.1 Telnet Encryption Protocol Security Considerations

The security consideration of the Telnet encryption protocol are
inherited.

It should be noted that the it is up to the authentication protocol
used, if any, to bind the authenticity of the peers to a specific
session.

The Telnet encryption protocol does not, in general, protect against
possibly malicious downgrading to any mutually acceptable, but not
preferred, encryption type. This places a requirement on each peer to
only accept encryption types it trust fully. In other words, the
Telnet encryption protocol do not guarantee that the strongest
mutually acceptable encryption type is always selected.

B.2.5.2 AES-CCM Security Considerations

The integrity and privacy claims are inherited from AES-CCM. In
particular, the implementation must make sure a nonce is not used more
than once together with the same key.

Furthermore, the encryption key is assumed to be random, i.e., it
should not be possible to guess it with probability of success higher
than guessing any uniformly selected random key. RFC 1750 gives an
overview of issues and recommendations related to randomness.

B.2.6 Acknowledgments

This document is based on the various Telnet Encryption RFCs (RFC
2946, RFC 2947, RFC 2948, RFC 2952 and RFC 2953).

B.3 Kerberized rsh and rlogin

This appendix describe the KCMDV0.2 protocol used in shishi patched
version of inetutils. The KCMD protocol was developped by the MIT
Kerberos team for kerberized rsh an rlogin programs. Differences
between rlogin an rsh will be explained, like those between v0.1 and
v0.2 of the protocol for compatibility reasons.
It is possible that some parts of this document are not in conformity
with original KCMD protocol because there is no official specification
about it. However, it seems that shishi implementation is compatible
with MIT's one.

B.3.1 Establish connection

First the client should establish a TCP connection with the
server. Default ports are 543 (klogin), 544 (kshell), 2105 (eklogin).
eklogin is the same as klogin but with encryption. Their is no longer
ekshell port because encrypted and normal connection use the same port
(kshell).
Kshell need a second connection for stderr. The client should send a
null terminated string that represent the port of this second
connection.
Klogin and eklogin does not use a second connection for stderr so the
client must send a null byte to the server.
Contrary to classic rsh/rlogin, server must not check if the client
port is in the range 0-1023.

B.3.2 Kerberos identification

When connections are established, first thing to do is to indicate
kerberos authentication must be used.
So the client will send a string to indicate it will used kerberos
5. It will call a length-string "strl" the couple (lenght of the string
strl, null terminated string strl). Length of the string is an int32
(32bits int) in MSB order (for the network).
So the client send this length-string strl :

KRB5_SENDAUTH_V1.0

After that the client must indicate which version of the protocol it
will used by sending this length-string strl :

KCMDV0.2

It can be V0.1 for older versions.
If indentification from client is good, server will send a null
byte (0x00). Else if authentication message is wrong, server send byte
0x01, else if protocol version message is wrong server send byte 0x02.

B.3.3 Kerberos authentication

When client is indentified, kerberos authentication can begin. The
client must send an AP-REQ to the server. AP-REQ authenticator must
have a subkey (only for KCMDV0.2) and a checksum.
Authenticator checksum is created on following string :

"serverport:""terminaltype""remoteusername"

for example :

543:linux/38400user

remoteusername corresponds to the identity of the client on remote machine.

AP-REQ is sended in der encoded format. The length (int32) of der
encoded AP-REQ is sended in network format (MSB), following by the der
encoded AP-REQ.
If all is correct, server send a null int32 (MSB format but like it is
null it is not important).
KCMD protocol use mutual authentication, so server must now send and
AP-REP : (in32 lenght in MSB of der encoded AP-REP)(der encoded
AP-REP).

Server must verify that checksum in AP-REQ authenticator is correct by
computing a new hash like client has done.

Server must verify that principal (in AP-REQ) has right to log in on
the remote user account.
For the moment shishi only check if remote user name is equal to
principal. A more complex authorization code is planned.
Look at the end to know how MIT/Heimdal do to check authorization.

If all is correct server send a null byte, else an error message
string (null terminated string) is sent. User read the first byte. If
it is equal to zero, authentication is correct and is logged on the
remote host. Else user can read the error messsage send by the server.

B.3.5 Window size

For rlogin protocol, when authentication is complete, the server can
optionnaly send a message to ask for window terminal size of
user. Then the user can respond but it is not an obligation.

In KCMDV0.1 server send an urgent TCP message (MSG_OOB) with one byte
:

TIOCPKT_WINDOW = 0x80

In KCMDV0.2 server does not send an urgent message but write on the
socket 5 bytes :

'\377', '\377', 'o', 'o', TIOCPKT_WINDOW

If encryption is enabled (eklogin) server must send this 5 bytes
encrypted.

Client can answer in both protocol version with :

'\377', '\377', 's', 's', "struct winsize"

The winsize structure is filled with corresponding setting to client's
terminal.
If encryption is enabled this answer must be send encrypted.

B.3.6 End of authentication

The "classic" rsh/rlogin can be used now.

B.3.7 Encryption

Encryption mode is used when a connection with eklogin is established.
Encryption with krsh can be used too. Before, there was a specific port
for that (ekshell), but now to indicate that encryption must be used with
krsh, client must add "-x " before the command when it send it between
remote user name and local user name.
When the client compute the checksum for AP-REQ authenticator the "-
x" must not be included.

Encryption in KCMDV0.2 is not the same as in KCMDV0.1.
KCMDV0.1 uses ticket session key as encryption key, and use standard
Kerberos encryption functions. This protocol only supports des-cbc-crc,
des-cbc-md4, des-cbc-md5 and does not use initialisation vectors.

For example on each encryption/decryption calls, the following
prototype kerberos function should be used :

kerberos_encrypt (key, keyusage, in, out) (or decrypt)

KCMDV0.2 can be used with all kerberos encryption modes (des, 3des,
aes, arcfour) and use AP-REQ authenticator subkey. In opposite to
KCMDV0.1 initialisation vectors are used. All encryptions/descryptions
must be made using a cryptographic context (for example to use the
updated iv, or sbox) :

For other KCMDV0.2 modes keyusage is different for each
encryption/decryption usage.
To understand, eklogin use 1 socket. It encrypts data (output 1) to
send and decrypts (input 1) received data.
Kshell use 2 sockets (1 for transmit data, 1 for stderr). So there are
four modes :

transmit : input 1
output 1
stderr : input 2
output 2

There is a keyusage for each modes. The keyusage must correspond on
client and server side. For example in klogin client input 1 keyusage
will be server output 1 keyusage.

I/O

Client

Server

intput 1

1028

1030

output 1

1030

1028

intput 2

1032

1034

output 2

1034

1032

Those keyusages must be used with AES and ARCFOUR modes.

KCMDV0.2 uses IV (initialisation vector). Like for keyusage, client IV
must correspond to server IV. IV size is equal to key type,
blocksize. All bytes of IV must be initialised to :

I/O

Client

Server

intput 1

0

1

output 1

1

0

intput 2

2

3

output 2

3

2

ARCFOUR mode does not use IV. However, like it is said before, a context
must be used to keep the updated sbox.

A check on message size can be made in second version of the protocol.

B.3.8 KCMDV0.3

This part only gives possible ways to extend KCMD protocol. Does not
take that as must have in KCMD implementation.

Extensions of KCMV0.2 could be made. For example kshell supposes there
are no files with name "-x *". I think the same thing can be supposed
with terminal name for klogin. So client could add "-x " to terminal
type it sends to server to indicate it will use encryption. Like that
there will be only one port for klogin/eklogin : 543.

In encrypted mode kshell send command in clear on the network, this
could be considered as insecure as user have decided to use
encryption.
This is not really a problem for klogin because it just sends terminal
type.

In encrypted mode, klogin and kshell clients could only send "-x" as
command or terminal type.
After that encryption is activated, and the client could send terminal
type or command encrypted.
The server will send the null byte to say that all is correct, or
error message in encrypted form.

B.3.9 MIT/Heimdal authorization

This part describes how MIT/Heimdal version check authorization of the
user to log in on the remote machine.

Authorization check is made by looking if the file .k5login exists on
the account of the remote user.
If this file does not exist, remote user name must be the same as
principal in AP-REQ to valid authorization.
Else if this file exists, check first verify that remote user or root
are the owner of .k5login.
If it is not the case, the check fails.
If it is good, check reads each line of that file and compare each
readed name to principal.
If principal is found in .k5login, authorization is valid, else user
is not allowed to connect on remote host with the specified remote
user name (that can be the same as principal).

So someone (for example user "user1") can remote log into "user2"
account if .k5login is present in user2 home dir and this file is owned
by user2 or root and user1 name is present in this file.

B.4 Key as initialization vector

The des-cbc-crc algorithm (see Cryptographic Overview) uses
the DES key as the initialization vector. This is problematic in
general (see below5), but
may be mitigated in Kerberos by the CRC checksum that is also
included.

From daw@espresso.CS.Berkeley.EDU Fri Mar 1 13:32:34 PST 1996
Article: 50440 of sci.crypt
Path: agate!daw
From: daw@espresso.CS.Berkeley.EDU (David A Wagner)
Newsgroups: sci.crypt
Subject: Re: DES-CBC and Initialization Vectors
Date: 29 Feb 1996 21:48:16 GMT
Organization: University of California, Berkeley
Lines: 31
Message-ID: <4h56v0$3no@agate.berkeley.edu>
References: <4h39li$33o@gaia.ns.utk.edu>
NNTP-Posting-Host: espresso.cs.berkeley.edu
In article <4h39li$33o@gaia.ns.utk.edu>,
Nair Venugopal <venu@mars.utcc.utk.edu> wrote:
> Is there anything wrong in using the key as the I.V. in DES-CBC mode?
Yes, you're open to a chosen-ciphertext attack which recovers the key.
Alice is sending stuff DES-CBC encrypted with key K to Bob. Mary is an
active adversary in the middle. Suppose Alice encrypts some plaintext
blocks P_1, P_2, P_3, ... in DES-CBC mode with K as the IV, and sends off
the resulting ciphertext
A->B: C_1, C_2, C_3, ...
where each C_j is a 8-byte DES ciphertext block. Mary wants to discover
the key K, but doesn't even know any of the P_j's. She replaces the above
message by
M->B: C_1, 0, C_1
where 0 is the 8-byte all-zeros block. Bob will decrypt under DES-CBC,
recovering the blocks
Q_1, Q_2, Q_3
where
Q_1 = DES-decrypt(K, C_1) xor K = P_1
Q_2 = DES-decrypt(K, C_2) xor C_1 = (some unimportant junk)
Q_3 = DES-decrypt(K, C_1) xor 0 = P_1 xor K
Bob gets this garbage-looking message Q_1,Q_2,Q_3 which Mary recovers
(under the chosen-ciphertext assumption: this is like a known-plaintext
attack, which isn't too implausible). Notice that Mary can recover K by
K = Q_1 xor Q_3;
so after this one simple active attack, Mary gets the key back!
So, if you must use a fixed IV, don't use the key-- use 0 or something
like that. Even better, don't use a fixed IV-- use the DES encryption
of a counter, or something like that.

B.5 The Keytab Binary File Format

The keytab file format is described in the file keytab.txt,
included in verbatim below.

The Kerberos Keytab Binary File Format
Copyright (C) 2006 Michael B Allen <mba2000 ioplex.com>
http://www.ioplex.com/utilities/keytab.txt
Last updated: Fri May 5 13:39:40 EDT 2006
The MIT keytab binary format is not a standard format, nor is it
documented anywhere in detail. The format has evolved and may continue
to. It is however understood by several Kerberos implementations including
Heimdal and of course MIT and keytab files are created by the ktpass.exe
utility from Windows. So it has established itself as the defacto format
for storing Kerberos keys.
The following C-like structure definitions illustrate the MIT keytab
file format. All values are in network byte order. All text is ASCII.
keytab {
uint16_t file_format_version; /* 0x502 */
keytab_entry entries[*];
};
keytab_entry {
int32_t size;
uint16_t num_components; /* sub 1 if version 0x501 */
counted_octet_string realm;
counted_octet_string components[num_components];
uint32_t name_type; /* not present if version 0x501 */
uint32_t timestamp;
uint8_t vno8;
keyblock key;
uint32_t vno; /* only present if >= 4 bytes left in entry */
};
counted_octet_string {
uint16_t length;
uint8_t data[length];
};
keyblock {
uint16_t type;
counted_octet_string;
};
The keytab file format begins with the 16 bit file_format_version which
at the time this document was authored is 0x502. The format of older
keytabs is described at the end of this document.
The file_format_version is immediately followed by an array of
keytab_entry structures which are prefixed with a 32 bit size indicating
the number of bytes that follow in the entry. Note that the size should be
evaluated as signed. This is because a negative value indicates that the
entry is in fact empty (e.g. it has been deleted) and that the negative
value of that negative value (which is of course a positive value) is
the offset to the next keytab_entry. Based on these size values alone
the entire keytab file can be traversed.
The size is followed by a 16 bit num_components field indicating the
number of counted_octet_string components in the components array.
The num_components field is followed by a counted_octet_string
representing the realm of the principal.
A counted_octet_string is simply an array of bytes prefixed with a 16
bit length. For the realm and name components, the counted_octet_string
bytes are ASCII encoded text with no zero terminator.
Following the realm is the components array that represents the name of
the principal. The text of these components may be joined with slashs
to construct the typical SPN representation. For example, the service
principal HTTP/www.foo.net@FOO.NET would consist of name components
"HTTP" followed by "www.foo.net".
Following the components array is the 32 bit name_type (e.g. 1 is
KRB5_NT_PRINCIPAL, 2 is KRB5_NT_SRV_INST, 5 is KRB5_NT_UID, etc). In
practice the name_type is almost certainly 1 meaning KRB5_NT_PRINCIPAL.
The 32 bit timestamp indicates the time the key was established for that
principal. The value represents the number of seconds since Jan 1, 1970.
The 8 bit vno8 field is the version number of the key. This value is
overridden by the 32 bit vno field if it is present.
The keyblock structure consists of a 16 bit value indicating the keytype
(e.g. 3 is des-cbc-md5, 23 is arcfour-hmac-md5, 16 is des3-cbc-sha1,
etc). This is followed by a counted_octet_string containing the key.
The last field of the keytab_entry structure is optional. If the size of
the keytab_entry indicates that there are at least 4 bytes remaining,
a 32 bit value representing the key version number is present. This
value supersedes the 8 bit vno8 value preceeding the keyblock.
Older keytabs with a file_format_version of 0x501 are different in
three ways:
1) All integers are in host byte order [1].
2) The num_components field is 1 too large (i.e. after decoding,
decrement by 1).
3) The 32 bit name_type field is not present.
[1] The file_format_version field should really be treated as two
separate 8 bit quantities representing the major and minor version
number respectively.
Permission to copy, modify, and distribute this document, with or
without modification, for any purpose and without fee or royalty is
hereby granted, provided that you include this copyright notice in ALL
copies of the document or portions thereof, including modifications.

The purpose of this License is to make a manual, textbook, or other
functional and useful document free in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible
for modifications made by others.

This License is a kind of “copyleft”, which means that derivative
works of the document must themselves be free in the same sense. It
complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does. But this License is not limited to software manuals;
it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book. We recommend this License
principally for works whose purpose is instruction or reference.

APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License. Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that
work under the conditions stated herein. The “Document”, below,
refers to any such manual or work. Any member of the public is a
licensee, and is addressed as “you”. You accept the license if you
copy, modify or distribute the work in a way requiring permission
under copyright law.

A “Modified Version” of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.

A “Secondary Section” is a named appendix or a front-matter section
of the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document's overall
subject (or to related matters) and contains nothing that could fall
directly within that overall subject. (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.) The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.

The “Invariant Sections” are certain Secondary Sections whose titles
are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License. If a
section does not fit the above definition of Secondary then it is not
allowed to be designated as Invariant. The Document may contain zero
Invariant Sections. If the Document does not identify any Invariant
Sections then there are none.

The “Cover Texts” are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License. A Front-Cover Text may
be at most 5 words, and a Back-Cover Text may be at most 25 words.

A “Transparent” copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters. A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart
or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount
of text. A copy that is not “Transparent” is called “Opaque”.

Examples of suitable formats for Transparent copies include plain
ASCII without markup, Texinfo input format, LaTeX input
format, SGML or XML using a publicly available
DTD, and standard-conforming simple HTML,
PostScript or PDF designed for human modification. Examples
of transparent image formats include PNG, XCF and
JPG. Opaque formats include proprietary formats that can be
read and edited only by proprietary word processors, SGML or
XML for which the DTD and/or processing tools are
not generally available, and the machine-generated HTML,
PostScript or PDF produced by some word processors for
output purposes only.

The “Title Page” means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page. For works in
formats which do not have any title page as such, “Title Page” means
the text near the most prominent appearance of the work's title,
preceding the beginning of the body of the text.

The “publisher” means any person or entity that distributes copies
of the Document to the public.

A section “Entitled XYZ” means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
text that translates XYZ in another language. (Here XYZ stands for a
specific section name mentioned below, such as “Acknowledgements”,
“Dedications”, “Endorsements”, or “History”.) To “Preserve the Title”
of such a section when you modify the Document means that it remains a
section “Entitled XYZ” according to this definition.

The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document. These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.

VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute. However, you may accept
compensation in exchange for copies. If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.

COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document's license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify
you as the publisher of these copies. The front cover must present
the full title with all words of the title equally prominent and
visible. You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.

If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.

MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it. In addition, you must do these things in the Modified Version:

Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document). You may use the same title as a previous version
if the original publisher of that version gives permission.

List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

State on the Title page the name of the publisher of the
Modified Version, as the publisher.

Preserve all the copyright notices of the Document.

Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document's license notice.

Include an unaltered copy of this License.

Preserve the section Entitled “History”, Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page. If
there is no section Entitled “History” in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on. These may be placed in the “History” section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

For any section Entitled “Acknowledgements” or “Dedications”, Preserve
the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.

Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers
or the equivalent are not considered part of the section titles.

Delete any section Entitled “Endorsements”. Such a section
may not be included in the Modified Version.

Do not retitle any existing section to be Entitled “Endorsements” or
to conflict in title with any Invariant Section.

Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled “Endorsements”, provided it contains
nothing but endorsements of your Modified Version by various
parties—for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled “History”
in the various original documents, forming one section Entitled
“History”; likewise combine any sections Entitled “Acknowledgements”,
and any sections Entitled “Dedications”. You must delete all
sections Entitled “Endorsements.”

COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.

AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an “aggregate” if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation's users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document's Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.

TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled “Acknowledgements”,
“Dedications”, or “History”, the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

TERMINATION

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense, or distribute it is void, and
will automatically terminate your rights under this License.

However, if you cease all violation of this License, then your license
from a particular copyright holder is reinstated (a) provisionally,
unless and until the copyright holder explicitly and finally
terminates your license, and (b) permanently, if the copyright holder
fails to notify you of the violation by some reasonable means prior to
60 days after the cessation.

Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.

Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License. If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.

FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License “or any later version” applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation. If the Document
specifies that a proxy can decide which future versions of this
License can be used, that proxy's public statement of acceptance of a
version permanently authorizes you to choose that version for the
Document.

RELICENSING

“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server. A
“Massive Multiauthor Collaboration” (or “MMC”) contained in the
site means any set of copyrightable works thus published on the MMC
site.

“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.

“Incorporate” means to publish or republish a Document, in whole or
in part, as part of another Document.

An MMC is “eligible for relicensing” if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

Copyright (C) yearyour name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the “with...Texts.” line with this:

with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.